The correct option is C Both A and R are true but R is not the correct explaination of A.
In large parts of the subcontinent, society was already divided according to the rules of varna. These rules, as prescribed by the Brahmanas, were accepted by the rulers of large kingdoms. The difference between the high and low, and between the rich and poor, increased. Under the Delhi Sultans and the Mughals, this hierarchy between social classes grew further. Here, in the question, the keyword is "further". Division on the basis of caste was present even before the rule of the Delhi Sultans and Mughals.
Hence, division of society on the basis of caste cannot be the main reason for the division to grow further. The divisions grew due to new administrative measures and the introduction of rank system.