Which of the Following Best Illustrates the Free-Rider Problem

Which of the following best illustrates the free-rider problem?

A) Individuals contribute toward a national defence program by paying taxes to the government.
B) Some homeowners in a mountain resort area refuse to contribute toward paving the area's only access road.
C) A tollbooth is constructed at the entrance of a privately funded highway.
D) A homeowner erects a locked gate around his swimming pool to keep out unsupervised children.