The correct answer is (a).
Population explosion is a rapid increase in the number of people in a population. It is caused by a high birth rate and a low death rate. In India, the birth rate is high because of cultural factors such as the importance of having children and the lack of access to family planning. The death rate is low because of improvements in healthcare and sanitation.
(b) is incorrect because a high birth rate and a high death rate would result in a stable population.
(c) is incorrect because a low birth rate and a high death rate would result in a declining population.
(d) is incorrect because a low birth rate and a low death rate would result in a slow-growing population.