Many major religions spread through
(a) people questioning old traditions. (b) people migrating from one place to another. (c) changes in philosophy and beliefs. (d) new ideas being introduced into a culture.

Well, consider each of the answers and see which one is how religions SPREAD.
A) people questioning old traditions, C)changes in philosophy and beliefs,  and D) new ideas being introduced into a culture all explain how a religion could change, or modify, not spread.

Therefore, we can see that B) people migrating from one place to another, would be the only way out of these that major religions could SPREAD.

The american civil war