Sugriva asked Rama to "help him to defeat his brother Bali."
Defeat refers to winning a battle over someone.
Because Sugriva wanted his kingdom back as he could not go and fight with his brother because if he went to his brother, he shall have half of his power. Thus, Sugriva could not defeat Bali, so he asked Rama to defeat his brother Bali.
Hence the correct option is b. "help him to defeat his brother Bali."
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