Which of these four religions claim the smallest number of adherents, a)christianity, b) hinduism, c) judaism, d)buddhism

The correct answer is - c) Judaism.

The Judaism is a religion that has its followers in multiple regions of the world, but is nowhere near the number of followers that the Christianity, Islam, Hinduism, and Buddhism have.

This religion is an Abrahamic religion, it is in fact the oldest of these type of religions, and is practiced mainly by the Jewish people. The estimates are that the Judaism is practiced by around 17 million people worldwide, with the basis being in Israel, but also in the northeastern part of the United States where there is a significant Jewish population.