"Universal set is the set that all the sets in study ares subsets"
Ok, so let´s consider this set:
$$U = \{A\, , \,B\, ;\, 1 \}$$
$$U = \{\,\{2\,;\,3\,\}\,;\,\{\,3\,;\,4\,\}\,;\,1\,\}$$
So $$\{2\,;\,3\,\} \in\,U$$ and $$\{2\,;\,3\,\}$$ is not a subset of $$U$$
Then, $$\{\,\{2\,;\,3\,\} \notin U$$ wich is wrong
Best Answer
$U$ is the set of all elements in $U$, which includes the elements in $A$ and in $B$.
Hence, $U = \{1, 2, 3, 4\}$, and so, although $2 \in U$ and $3 \in U$, it is not the case that $\{2, 3\} \in U$, but rather, $\{2, 3\} \subset U$.