Creativity is the answer.
They are territory of the United States, but not considered a state.
It is definitely not true that many small rural communities have best transport, so we can reject the option b) and d) together with it.
The best answer is that they don't have mass transit systems (a is the correct answer). It's not true that they can walk everywhere because they can for example be forced to walk to a different town if they need a hospital.
