As said by Chris in comments, you can use minipages. To have caption and not to allow floating, you can avoid figure environment and use \captionof macro from either caption or capt-of packages.
\documentclass[a4paper,11pt]{article}
\usepackage{standalone}
\usepackage[singlelinecheck=off,hang]{caption} %% provides \captionof command. capt-of package does this too.
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\usepackage{filecontents}
\begin{filecontents*}{picture1.tex}
\documentclass{standalone}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\pagestyle{empty}
\begin{document}
\begin{tikzpicture}[line cap=round,line join=round,>=triangle 45,x=1.0cm,y=1.0cm]
\draw [domain=0.08:3.92] plot(\x,{(--1.04--0.32*\x)/1.96});
\draw (3.6,0.98) node[anchor=north west] {$l_a$};
\end{tikzpicture}
\end{document}
\end{filecontents*}
\begin{filecontents*}{picture2.tex}
\documentclass{standalone}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\pagestyle{empty}
\begin{document}
\begin{tikzpicture}[line cap=round,line join=round,>=triangle 45,x=1.0cm,y=1.0cm]
\draw [domain=0.08:3.92] plot(\x,{(--1.04--0.32*\x)/1.96});
\draw (3.6,0.98) node[anchor=north west] {$l_b$};
\end{tikzpicture}
\end{document}
\end{filecontents*}
\begin{document}
\begin{minipage}[t]{.45\textwidth}
\includestandalone{picture1}
\captionof{figure}{Some description of figure comes here}
\end{minipage}%
\hfill
\begin{minipage}[t]{.45\textwidth}
\includestandalone{picture2}
\captionof{figure}{Some description of }
\end{minipage}
\end{document}
Or you can use subfigure from subcaption package. In this case you will need float package that provides [H] position specifier.
\documentclass[a4paper,11pt]{article}
\usepackage{standalone}
\usepackage[singlelinecheck=off,hang]{caption} %% for formatting captions
\usepackage{subcaption}
\usepackage{float} %% for controlling float position, provides [H] postion specifier.
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\usepackage{filecontents}
\begin{filecontents*}{picture1.tex}
\documentclass{standalone}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\pagestyle{empty}
\begin{document}
\begin{tikzpicture}[line cap=round,line join=round,>=triangle 45,x=1.0cm,y=1.0cm]
\draw [domain=0.08:3.92] plot(\x,{(--1.04--0.32*\x)/1.96});
\draw (3.6,0.98) node[anchor=north west] {$l_a$};
\end{tikzpicture}
\end{document}
\end{filecontents*}
\begin{filecontents*}{picture2.tex}
\documentclass{standalone}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\pagestyle{empty}
\begin{document}
\begin{tikzpicture}[line cap=round,line join=round,>=triangle 45,x=1.0cm,y=1.0cm]
\draw [domain=0.08:3.92] plot(\x,{(--1.04--0.32*\x)/1.96});
\draw (3.6,0.98) node[anchor=north west] {$l_b$};
\end{tikzpicture}
\end{document}
\end{filecontents*}
\begin{document}
Some text comes here
\begin{figure}[H]
\begin{subfigure}[t]{.45\textwidth}
\includestandalone{picture1}
\caption{Some description of figure comes here}
\end{subfigure}%
\hfill
\begin{subfigure}[t]{.45\textwidth}
\includestandalone{picture2}
\caption{Some description of figure}
\end{subfigure}
\end{figure}
\end{document}
The same can be achieved with subfig package also. This is left as an exercise.
you've already separated the different elements, providing spaces between the distinct equations comprising each language and separately coding these equations as math (even
though the space between the first two is, probably inadvertently, omitted).
unfortunately, these spaces don't fall in a place that is optimal for tex to break the line.
the ultimate goal is for what is presented to be understood.
there are two parts to this recommendation.
first, the words "milk, curry, rice" are, as you say, constants, and as such should be
in a text font, preferably not italic in this context, even though they're part of the
math expression. as coded in your original, they are typeset as strings of variables
multiplied together. these could be coded as \mathrm{<word>}, but that doesn't help
with line breaking. it also wouldn't leave spaces after the commas, although in this
situation, whether spaces are visible there or not wouldn't be misunderstood by a reader.
another way to approach these is to recognize them as text, and input them as, for example,
$\mathcal{C}_o=\{\text{milk, curry, rice}\}$
but this doesn't help with line breaking either, since in this context, the only
"allowable" break is after the equals sign.
so, second part of suggestion, take advantage of the fact that a reader isn't likely
to misunderstand what is meant if a line is broken within that string of constants,
and (temporarily) terminate the math after the opening brace, and reinstate it for the
ending brace:
$\mathcal{C}_o=\{$milk, curry, rice$\}$
to illustrate, using a forced line break for the "all math" instance, compare these
two lines:
here's the input that produced the image:
\begin{itemize}
\item The language of enquiry $\mathcal{L}$ is given by
$\mathcal{C}_o=\{milk, curry, rice\}$,\\
$\mathcal{R}_o=\{TastesHot, IsWhite, ContainsSpice, ContainsSugar\}$,
$\mathcal{F}_o=\{\}$.
\item The language of enquiry $\mathcal{L}$ is given by
$\mathcal{C}_o=\{$milk, curry, rice$\}$,
$\mathcal{R}_o=\{$TastesHot, IsWhite, ContainsSpice, ContainsSugar$\}$,
$\mathcal{F}_o=\{\}$.
\end{itemize}
(by the way, that's hardly a minimal example.)
Best Answer
Move the
to the first line after
\begin{minipage}to avoidminipageoverrides them.