Answer:
(C)
Explanation:
Premiums is something given as a reward, prize, or incentive.
In this case, the college entertainers offered an incentive or reward (premiums) to the people who came to see them perform.
The shirts would encourage the people who came to see them perform to be present some other time.
For example, a grocery store may give free sweets to children who accompany their parents to the store.