Popular sovereignty was a policy stating that
a. voters in a territory should decide whether or not to allow slavery in that territory.
b. congress should decide whether or not to allow slavery in a territory.
c. voters in a territory should decide whether or not to join the union.
d. voters in a state should decide whether or not to stay in the union.

I think that the question is in the context of the Kansas–Nebraska Act, from 1854.  (Popular sovereignty is a more general principle that says that the power is with the people - this would mean that options a, c and d would be true.).

But in the context of the Kansas–Nebraska Act the correct answer is that people could decide on slavery - the correct answer is A.