isro cs question paper solution 2018 Q25 Set-A

CFG (Context Free Grammer) is not closed under

(a) Union
(b) Complementation
(c) Kleene star
(d) Product

Answer is:  (b) Complementation

Intersection and complementation of two CFG need not be context free.

Closed under union, concatenation, and Kleene star.
Closed under substitution, homomorphism, and reversal