The correct answer is B.
After spending time with the Houyhnhnms, which were rational horses who controlled the land, Gulliver becomes fascinated with them and wanted to become one of them.
In comparison with these magnificent creatures, the Yahoos (savage humanoid creatures) seemed despicable to him.
So his attempts to imitate the Houyhnhnms led Gulliver to hate human beeings, including himself.