Many small rural communities
a. do not have mass transit systems
c. are small enough to walk everywhere
b. have the best mass transit systems
d. all of the above

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.