Not necessary, because the author’s argument does not rely on any claims about modern ground-feeding animals. Even if the same type of neck structure is NOT found on modern ground-feeding animals, those animals could simply have different body features that allow them to eat ground-level food.

Not necessary, because the argument doesn’t depend on any claims about what Diplodocus was able to see.

Not necessary, because even if it’s possible for a large animal like Diplodocus to supply blood to an elevated brain, the reason given for why the Diplodocus didn’t feed on high-growing vegetation has nothing to do with inability to supply blood to the brain. The reason is based on the structure of the animal’s neck bones.

Necessary, because if it were not true — if Diplodocus DID have other ways of accessing high-growing vegetation, such as by rising up on its hind legs — then the fact the animal’s neck couldn’t raise into the air wouldn’t prove that Diplodocus didn’t feed on high-growing vegetation. The negation of (D) would raise the possibility Diplodocus could have fed on high-growing vegetation.

Not necessary, because the negation of (E) actually helps the argument. If the Diplodocus COULD browse underwater vegetation by kneeling besides water, that provides another reason to believe it may have fed on plants underwater.