Step 1: Understanding the context of the quote.
K. M. Kapadia, a renowned sociologist and anthropologist, stated that "Hindu marriage is a religious sacrament." He emphasized that Hindu marriage is not merely a social contract but a sacred and religious rite, integral to the cultural and spiritual fabric of Hindu society.
Step 2: Conclusion.
K. M. Kapadia is credited with defining Hindu marriage as a religious sacrament, highlighting its significance beyond just a social institution.
Final Answer: \[ \boxed{\text{The correct answer is (C) K. M. Kapadia.}} \]