Which of the following Grammars is/are only Context Free (i.e. context-free but not regular grammars)?

Grammar	Productions
A	S → Ab aS → aA A → a
B	S → Ab A → Sa A → a
C	S → AS S → aA A → a
D	S → Ab S → aA A → a
E	S → Sb A → Aa A → ε

Explanation

A has the rule aS → aA, whose left side has more than one symbol, so it is context-sensitive, not context-free. E (S → Sb, A → Aa, A → ε) is entirely left-linear, so it is a regular grammar. C has S → AS (two non-terminals on the right) and D mixes a left-linear rule S → Ab with a right-linear rule S → aA, so neither is a regular grammar; both are only context-free. (B is also context-free, but no option pairs it correctly, so C, D is the answer.)