The correct answer is - b. Christianity.
There are lots of religions in the world, some of which are global religions, practiced by a very large number of people. The largest percentage of the world's population is practicing an abrahamic religion, the Christianity. With an estimated 2.42 billion followers, the Christianity is occupying the number one spot, and has over 600 million followers more than the second placed Islam.
Even though it still holds the number one spot, the Christianity will probably not be the dominant religion in the world in few decades, and this is due to the trends in it, as well the trends in the second placed Islam. Namely, the Christianity is losing followers, as big portion of the younger generations are becoming atheists, and also, because of the low fertility rates. On the other hand, the Islam is having a growing following base, and the youngsters of this religious background don't tend to abandon the religion, and also there's a high fertility rate in this communities, so it is expected that the Islam will become the most followed religion in the world in the coming decades.