How did people in India move between castes?

The third slot went to the Vaishyas, or the traders, who were created from his thighs. At the bottom of the heap were the Shudras, who came from Brahma's feet and did all the menial jobs. The main castes were further divided into about 3,000 castes and 25,000 sub-castes, each based on their specific occupation.

the correct answer is d

i think it would be a