For a)
L = { $a^{0}, a^{2}, a^{4}, a^{6} . . .$ }, the number of a's are in Arithmetic Expression. If Language is in AP then it is always Regular.
For b)
Language generated by CFG is CFL.
CFL $\cap$ CFL = is not necessarily CFL. But emptiness problem ( L = $\emptyset$ ? ) is DECIDABLE only for Regular and CFL. Therefore, answer to (b) is Undecidable.