For this question we need to analyze the argument's structure. The portion of the argument they are asking us about is the conclusion, so this should be straightforward. If something is a conclusion, it is not evidence, an intermediate conclusion, a fact, or an assumption. This should make it pretty straightforward to eliminate (C). The others use "conclusion" or "generalization" both of which can indicate a conclusion. Looks like we'll have to sort these more finely.
(A) indicates the wrong premise supporting the conclusion - the conclusion is based more on the organisms' ability to evolve and not so much on their being numerous - this has a scope issue, so it's out.
(B) indicates the correct premise upon which the argument relies - we can see this (other than the conclusion itself) in particular "they quickly evolve immunities to those medicines."
(D) looked promising at first - conclusions can be generalizations, but when we get more into the text here, we see that "generalization" is being used as a premise not as a conclusion here (since it says it is used "to predict." This is out.
(E) is close - quite tempting - but no cigar. It's not the immunity but rather the evolution that supports the strong wording of the conclusion that we will *never* be free (i.e. no matter how good the medicines get).
So (B) is our answer. Any questions about this one?