A talented musician plays for tips on the street but never seems to make very much money. Which of the following economic concepts would explain why his tip jar is never very full?

The correct answer is: the free-rider problem.

Explanation:

The free-rider problem occurs when individuals do not want to pay their fair share for something others pay. The free-rider problem tends to happen when everyone has access to a source without constraints and there is little to no regulation over the use of the resource.

Thus, the talented musician's jar is never full because of the free-rider problem: some people give the musician tips to eager him to continue doing it or because they liked the music while others, even if they liked and enjoyed the musician's play, are reluctant to tip him since they prefer to listen to the music for free.