Answer choice A is describing something that never happened. First, keep in mind that the conclusion the author is attacking is the one about the radio station. The political part is just a made-up example to support their point. So how did the radio station come to its conclusion? By judging from who called in. There's no mention of a survey, whether by a biased party (the radio station) or someone else (a market research company, etc.).
Even if we did want to point out flaws in the example case, notice that we're never told who's conducting this hypothetical survey, either. Sure, the responders may be biased, but that's not what A says. (And a survey isn't wrong because the respondents are biased--we want to find out what they think, biases and all!)
Finally (and this may be the most important point), we can't choose A just because
we think bias is involved. The question is asking how the argument proceeds, so we need to address what the
author does. Since they never directly say that the surveyors are biased, that can't be how the argument proceeds. However, they do directly use a flawed example to show why the original argument is wrong, so B works.