The correct answer is - D. India.
As of 2010, the percentage of the Hindu followers in the South Asian country of India was reaching 80%. It is by far the most dominant religion in the second most populous country in the world, and it is set to be the most dominant one for a long time into the future. Even the Muslims in the country have a slight rise every year, the sheer number of the Hindu population, and the fact that they also have a high fertility rate, puts them on top.
The Hinduism is a religion that can not really be considered as a global religion, even though it is in the top religions by the number of followers, but instead it is more of an ethnic religion, as it is only practiced in India.