inclusion and concatenation of languages
The assignment tests basic definitions (and some elementary set theory).

As you know, for arbitrary $L_1,L_2$, you have $L_1\cdot L_2 = \{ x\cdot y \mid x\in L_1, y\in L_2\}$. 

Now if $L_1\subseteq L_3$ and  $L_2\subseteq L_4$ then $L_1\cdot L_2 \subseteq L_3\cdot L_4$. This is rather clear, or if you want, can shown with elementary steps. Take any $z\in L_1\cdot L_2$, then $z=xy$ for $x\in L_1$ and $y\in L_2$. However, because of the inclusions, $z=xy$ for $x\in L_3$ and $y\in L_4$, thus $z\in L_3 \cdot L_4$.