Not necessary, because even if Neanderthals are NOT “known” to have possessed the dexterity required to create paintings like the one discovered, that doesn’t imply that they did not have the requisite dexterity.

Not necessary, because even if SOME species of hominid other than Neanderthals inhabited some part of Europe 40,800 years ago, Neanderthals still could have been the only hominids in northern Spain. Other hominids could have existed elsewhere in Europe; that doesn’t undermine the belief that Neanderthals likely made the painting found in northern Spain. Notice also that the author’s intermediate conclusion just says that Neanderthals “likely” made the painting; so the author could acknowledge another hominid might have made the paintings.

Necessary, because if this were not true — if the ability to create cave paintings like the one found does NOT indicate the ability to think symbolically — then we have no reason to think that the painting suggests Neanderthals could think symbolically.

Not necessary, because the negation of (D) doesn’t undermine the author’s reasoning. If there was other evidence besides the painting that suggests Neanderthals could think symbolically, that, if anything, would strengthen the author’s view.

Not necessary, because the argument tries to use evidence to reach the conclusion that Neanderthals could think symbolically. That doesn’t require any assumption about what would show that hominid could NOT think symbolically.