- Magnetic fields from power lines can cause health problems to people living close by.
- Far away homes have stronger magnetic fields (caused by something else at home) than the strength of magnetic field close to power lines.

Conclusion: It is unlikely that magnetic fields from power lines cause health problems to people living close by.

Mind you, we are talking about different kinds of magnetic fields - those generated by power lines and those generated at home by other sources. We are saying that magnetic fields from power lines are weaker in people's homes that are close by. Stronger magnetic fields exist in far away homes. So it is unlikely that magnetic fields from power lines can cause health problems (because they are weaker) and stronger fields already exist.

Which of the following is useful to evaluate:

(A) Whether magnetic fields in homes that are not located near high-voltage power lines can cause health problems for the residents of those homes

We are asking whether the magnetic fields at home are harmful. If they are stronger and harmful like the magnetic fields of power lines, then it is true that weak fields of power lines may not be a source of worry. But say that magnetic fields of home at harmless - then it doesn't matter how strong they are, they will not cause any worries. The weaker fields of power lines may be a source of worry then because even the weak fields may cause health problems.
Basically, we are trying to find out whether the two types of magnetic fields are comparable. If they are not similar, then the argument may not hold since the argument is based on this comparison.

(B) What proportion of people who live near high-voltage power lines believe that those lines may cause health problems

People's beliefs have nothing to do with what actually happens. Irrelevant.

(C) Whether high-voltage power lines are routed near residential dwellings in urban areas.

Whether residential areas are near power lines is irrelevant. We want to find out what happens when they are close to power lines.
So our argument is what happens when A happens? Whether A actually does happen or not is irrelevant.

(D) What specific diseases or conditions are suspected by some researchers to result from exposure to magnetic fields from high-voltage power lines

The actual diseases are irrelevant. We are talking about all health problems.

(E) What is the average strength of magnetic fields in workplaces that are not located near high-voltage power lines

Our argument talks only about homes, not workplaces.

Answer (A)