Step 1: Understanding the Concept:
The question asks for the primary reason for India's high population growth. Overpopulation occurs when a high birth rate is combined with a declining death rate. The question focuses on the causes of the high birth rate.
Step 2: Detailed Explanation:
- (B) Socio-cultural factors: This is the most comprehensive cause. It includes a range of factors that promote high fertility, such as the universality of marriage, early age of marriage, preference for male children (leading to more births until a son is born), religious beliefs that favor procreation, and lack of awareness or acceptance of family planning.
- (C) Fecundity refers to the biological potential for reproduction. While necessary for population, it is constant across societies and does not explain why one society has a higher birth rate than another. The actual birth rate (fertility) is what matters, and it is shaped by social factors.
- (D) Modernity is generally associated with a \textit{decrease} in population growth through education, urbanization, and changing values (demographic transition).
Step 3: Final Answer:
The main cause of overpopulation in Indian society is a complex set of socio-cultural factors that sustain a high birth rate.