Necessary, because if this were not true — if NONE of the crows that attacked the people wearing the masks were not among the crows that had been trapped (in other words, if every one of the crows were part of the crows that had been trapped years before) — then we have no support for the claim that crows “can pass their concerns on to other crows.” If the attacking crows were the same ones that were caught before, that raises the possibility that they did not pass their concerns to other crows — they might have simply remembered the masks that caught them before and reacted to those masks.

Not necessary, because even if crows that perceive an individual as threatening do NOT always respond by shrieking and dive-bombing, it can still be the case that any crow that responds by shrieking and dive-bombing perceives an individual as a threat. Another way to put it is that the author assumes “shriek and dive-bomb → perceives as threatening.” (B) reverses this assumption.

Not necessary, because the argument concerns only how crows behave and what crows are capable of. What other bird species do has no effect on the reasoning, which is based only on observations of crows.

Not necessary, because (D) arguably weakens the argument if assumed. If most crows will attack people wearing caveman masks, even in places where crows have never been captured, that suggests attacking people wearing caveman masks may simply be a common reaction of crows, and not necessarily evidence of “recognizing” people who are threatening.

Not necessary, because the study did not involve people showing their unmasked faces. So whether crows can “recognize individual human faces” has no bearing on the reasoning. Even if crows can recognize individual human faces, that doesn’t undermine the argument.