Correct option is A. Hinduism
The rise of Rajput clans to the position of rulers set an example for the tribal people to follow. Gradually, with the support of the Brahmanas, many tribes became part of the caste system. But only the leading tribal families could join the ruling class. A large majority joined the lower jatis of caste society. On the other hand, many dominant tribes of Punjab, Sind and the North-West Frontier had adopted Islam quite early. They continued to reject the caste system. The unequal social order, prescribed by orthodox Hinduism, was not widely accepted in these areas.