This best captures the author’s support for the hypothesis that the pronghorn’s speed is an adaptation to extinct predators. This hypothesis is expressed at the end of P1 and supported by P3 and P4.

The author doesn’t express doubt about the hypothesis that the pronghorn’s speed is an adaptation to extinct predators. The reference to arctic ground squirrels at the end of the passage is offered to show that relict behavior might not exist forever. It doesn’t show that relict behaviors don’t exist.

There’s no support for the claim that “most present-day characteristics” of animals are explained by environmental conditions that don’t exist anymore. Although the author does discuss some examples of relict behavior, she never suggests “most” present-day characteristics are relict behavior. Since (C) is not supported, it can’t be the main point.

Although the author might agree with (D), it is not the main point. The main point is that there’s support for the hypothesis that pronghorns’ speed is an adaptation to extinct predators. The reference to arctic ground squirrels at the end of the passage does suggest that relict behaviors won’t exist forever, but this is merely a side point that isn’t part of the author’s main argument.

The author doesn’t suggest that there is “widespread agreement among biologists” that many characteristics are adaptations to extinct predators. Also, (E) doesn’t capture the author’s opinion about what explains pronghorns’ speed. The passage is about a particular phenomenon (pronghorns’ speed) and a hypothesis for it. The main point isn’t about “many animal characteristics.”