If we see the conclusion as “Crabs evolved UV vision to hunt glowy prey,” (A) strengthens (booo!) the argument by eliminating the alternate explanation (booo!) that the crabs evolved UV vision for no reason at all.

This is a Weaken question. (A) has other problems too, but this one’s the biggest and easiest to catch.

Taking (E) out of the picture for a moment, (B) does in fact weaken this argument (a little bit).

On its strongest interpretation, (B) prompts us to imagine a world where there is no prey, so crabs get their food not from hunting but from something else – like scavenging. If crabs don’t hunt at all, their UV vision can’t be for the purpose of hunting. It must be for something else. We’ve poked a hole in the offered explanation.

The bones of this logic are legit. This “crabs scavenge” scenario is a possible world where our conclusion isn’t true, and (B) takes us a step closer to that world by suggesting there isn’t much prey around.

But (B) suffers from a few limitations that make it weaken the argument only a little, and certainly less than (E). First is the weak wording of “not plentiful.” Our strongest reading of (B) used the suggestion that there is no prey down there at all. But (B) really suggests something more like “prey is hard to find down there,” which, if left at that, might increase the need for UV vision – prey is super scarce, so any conceivable advantage would be huge.

So saying (B) pokes a hole in the offered explanation is a bit much. It’s more like (B) gestures vaguely to a part of the argument that could conceivably get a hole poked in it.

Second, (B) stops short of providing a plausible alternative explanation for the crabs’ UV vision. (B) reduces our confidence in the explanation we’re given – it makes us think “hmm, maybe the crabs’ UV sensors aren’t for prey, but for… something else I dunno what” – while (E) points directly at a different glowy creature that would suffice to motivate crabs to see in UV on its own.

Weaken questions do sometimes force us to compare two answer choices that weaken the argument somewhat. Both the above reasons help to separate (E) cleanly from (B).

(C) can be seen most directly as a trap for people who don’t have the argument’s actual conclusion firmly in mind. Like if the stimulus ended with “…since the crab is not itself bioluminescent, it probably can’t see other crabs,” (C) would match. But, you know, the actual conclusion is that the crabs hunt glowy prey.

Judging (C) in the framework of the stimulus’ actual argument, it underscores the idea that the crabs’ UV eyes aren’t for spotting fellow crabs. Two problems here: that idea doesn’t require support since the premises already tell us crabs don’t glow, and what are we doing supporting claims in the argument anyway?!? This is a WEAKEN question!

(D) suggests the crabs’ UV eyes aren’t new. It would weaken the argument if the proposed explanation were about some novel factor that just recently arose in the crabs’ habitat.

Like if the argument said “the crabs' ultraviolet receptors likely developed because a cool UV sign sunk down there last week and they all wanted to see it.”

But there’s nothing in the proposed “crabs use UV vision to hunt glowy prey” explanation to suggest this phenomenon is recent. Crabs have always needed to eat.

This presents a clean alternate cause: maybe the crabs’ UV receptors aren’t for hunting glowy prey, but instead for avoiding glowy predators.

There’s room in (E)’s world for both things to be true (maybe crabs use their UV vision to hunt glowy prey and avoid glowy predators) but that’s kinda the point – the argument assumes hunting prey is the only plausible explanation when in reality there are other uses for UV vision the argument doesn’t consider.