Let's take some string:
$S\rightarrow aAb\rightarrow aaAbb\rightarrow aaBbb\rightarrow aaCCbb\rightarrow aabDaCbb\rightarrow aabDabDabb\rightarrow aabababb\rightarrow a^2b^1a^1b^1a^1b^2$
Option B and D eliminated.
$S\rightarrow aAb\rightarrow aBb\rightarrow aCCb\rightarrow abDaCb\rightarrow abDabDab\rightarrow abbDaabDab\rightarrow abbaabDab\rightarrow abbaabab\rightarrow a^1b^2a^2b^1a^1b^1$
Option C eliminated.
$Ans:A$