PLEASE HELP!! what is the best definition of the caste system

a. the hindu belief that life cycle is a cycle of birth, death, and rebirth, which ends only when a person reaches nirvana

b. an integral part of hinduism that teaches that the gods will allow people who pray to the wheel of dharma to change their social standing

c. a teaching of siddhartha gautama based on his realization that the only way to avoid suffering is to follow the eightfold noble path

d. social and religious classes developed after aryans settled on the ganges plain and later incorporated into hindu beliefs