eliminate one by one
for D :S-->aB-->ab (GENERATES #a = #b) SO IT IS WRONG
FOR A: CONSIDER THE STRING aaab (ODD NUM OF a'S AND ODD NUM OF b'S)
S-->aB-->aaBB-->aaaBBB-->aaabbb ( WE ARE NOT ABLE TO GET)
FOR B: CONSIDER THE STRING aaaabb ( EVEN NUM OF a'S AND EVEN NUM OF b'S)
S-->aB-->aaBB-->aaaBBB-->aaaaBBBB-->aaaabbbb(WE ARE NOT ABLE TO GET)
SO A,B,D ARE WRONG
NOW LOOK AT THE OUTPUTS OF A,B,D ..THESE ARE: ab,aaabbb,aaaabbbb
which shows we can get #a's = # b's
so C is the ans