The passage tells us that if the library shared by the towns of R & G is relocated to central G, it would be within walking distance of a larger number of library users. That is because there are more people living in central G, and people will walk to the library only if it is close to their homes.
(B) strengthens this argument because it tells us that the library is going to be serving the area of the same size as previously, but as there are more people living in this area, more people will be living within the walking distance of the library. Let's consider the opposite - if central G covered an area larger than central R, then it is likely that some people would be too far to be within walking distance, and even though there are more people living in the area, we could not conclude that the new building would be "within walking distance" of a larger number of library users. That is why this assumption strengthens the argument's conclusion.
