The argument is about whether the government should fund campaigns, not about whether candidates should run for office. The point of the argument is that the government should fund elections so that candidates don’t have to compromise their principles.

Like (A), (B) is talking about what candidates should do, whereas the argument is talking about what the government should do.

(C) is talking about what voters should do, whereas the argument is talking about what the government should do. The argument doesn’t say anything about how voters should choose a candidate to support.

This is key. If the government shouldn’t finance all activities that further an interest of society, then it’s invalid to conclude that the government should finance campaigns just because it will further an interest of society. (D) helps justify the reasoning in the argument.

(D) sets up a scenario where private funding should be encouraged — but the argument doesn’t do the same. The argument is saying that the government should finance campaigns, it’s not saying that there are special cases where private funding should be encouraged.