1.Does a given program ever produce an output?
Here 'Yes' ans is possible but 'No' ans not possible. So, it is Recursive enumerable
2 .If L is a context-free language, then, is L' also context-free?
As context free language is not closed under complementation, So, CFL not decidable under complementation
3.If L is a regular language, then, L' is also regular?
Regular language closed under complementation. So, it is decidable
4.If L is a recursive language, then, is L'also recursive?
Yes it is decidable
Recursive language is decidable
When the language complemented, the 'Yes' and 'No' ans is also complemented. i.e. 'Yes' becomes 'no' and 'no' becomes 'Yes'. But in complementation of Recursive language we also get 'Yes ' and 'No' answer. So,It is decidable
So, answer is (D)