$S \to XY$
$X \to aX \mid a$
$Y \to aYb \mid \epsilon$
$X$ generates atleast one '$a$'. While $Y$ generates equal no of $a$'s and $b$'s( including epsilon).
$L = \{ a , aa, aaa, aab, aaaa, aaab,aaaaa, aaabb, \ldots\}$
Hence, answer should be Option C.