(A) presents a different phrasing of our predicted answer, and therefore guarantees the conclusion. (A) presents a choice between randomness and non-randomness, which is triggered by the evidence some genetic mutations are random to yield a conclusion that all genetic mutations must be random.

The bacteria in the experiment being common doesn't make the argument hold water. The argument's flaw of over-generalization still exists regardless of whether the bacteria used were common or rare. Being common doesn't make these bacteria representative of all life-forms.

(C) is tempting, but when reading closely, the argument doesn't trigger (C) because the sufficient condition presented by (C) isn't met. The argument doesn't actually establish that all genetic mutations in bacteria are random, only that some genetic mutations in bacteria are random. Because (C) isn't triggered, it can't bridge to the conclusion that all genetic mutations are random.

Fundamentally, the argument's flaw of over-generalization would still be present even if the conclusion were limited to bacteria. The argument only establishes that genetic mutations in bacteria are sometimes random—for all we know, other bacteria in other conditions would have non-random mutations. We can't make the leap to assume that all genetic mutations in bacteria are random, because the argument's support doesn't even get us that far.

(D) provides one piece of the puzzle, but it doesn't go far enough to guarantee the conclusion of the argument. Even if the environment doesn't make a difference, what about the type of organism? The argument doesn't consider any examples except bacteria, so that's another variable the correct answer needs to account for. Compared to a more comprehensive assumption like (A), (D) still leaves gaps in the argument.

The author is claiming that all genetic mutations are always random, so distinguishing between nature and artificial environments doesn't make a difference. If (E) were true, but there were also artificial nutrients that caused non-random mutations, that would still disprove the argument. We can see from that possibility that (E) doesn't do enough given the absolute nature of the conclusion.