Fact: More people live in center G than in central R.
Premise: People will walk to the library only if it is locted close to their homes.
Conclusion: Library building in central G is within walking distance of a larger number of users.
To get to the conclusion we have to prove that more people live within walking distance to the library. If the two area are the same size, then the higher density area would generate more people within walking distance, no matter how walking distance is defined. Therefore B is correct.
E is out of scope, and actually weakens the argument by defuse the premise by stating people is willing to walk to the library even if it is located farther then what is normally considered walking distance.
Good one honghu, I failed to get the proper conclusion for this passage.