why not B?

It cant be B because B says "Wireless networks work far better where population density is low" means villages/small towns. This statement is not supporting wireless network for cities whereas argument says that cities need wireless network as well. Hence only option A holds.

We need to find a statement that is most likely cited by a supporter of the argument. The argument states that implementing wireless networks in the city would be a practical way to meet needs. This is because rural communities have found that wireless networks are both more reliable and cheaper than land-based networks.

A- Correct. This is something proponents of the argument will pose since it negates the possibilities of any weakeners because of a change in location.
B- This weakens the argument since its saying wireless networks won't work as well in urban areas. Incorrect.
C- Irrelevant to talk about Iceland.
D- Again weakens the argument by stating a disadvantage to installing wireless networks in urban areas.
E- Access to internet is irrelevant here.

(A) Urban areas do not pose additional problems for the effective operation of wireless networks. - Correct, assumption is it will be as effective in urban as rural & there is no difference between both these areas
(B) Wireless networks work far better where population density is low. - Irrelevant - no where do we speak about population density
(C) Iceland, a very rural country, successfully uses wireless networks. - Out of scope
(D) The expense of wireless transmission in areas with large buildings is much higher than in areas without such buildings. - Irrelevant we talk about urban/rural not large/small building
(E) Poor neighborhoods have less access to internet services than do educators or businesses.- Irrelevant as this has nothing to do with urban/rural comparison

Updated