For any non-trivial $A,B$, finding a language which both are polynomially reducible to
Part (a) is correct since you can take the language
$$ C = \{ 0x : x \in A \} \cup \{ 1y : y \in B \}. $$
Here $0x$ is the concatenation of the strings 0 and $x$. This part works even when $A,B$ are trivial.

Part (c) is correct by taking $C = \emptyset$ and using $A,B \neq \Sigma^*$, for example.