The fact that tooth and body size weren’t exactly the same among adult mammoths tells us nothing about whether tooth and body size were proportional to each other in mammoths. Because of this, (A) doesn’t guarantee that mammoths on the island were smaller.

This suggests that the teeth were not smaller due to natural wear. This might strengthen the argument, but it doesn’t guarantee the conclusion. Just because natural wear didn’t change the size of the mammoths’ teeth does not prove that the mammoths themselves were smaller.

The author is only comparing adult mammoth teeth on the island and adult mammoth teeth elsewhere. The size of juvenile mammoth teeth is irrelevant.

If tooth size in adult mammoths was always directly proportional to their overall size, this guarantees that the mammoths on the island were smaller than other mammoths, since their teeth were 25% smaller than other mammoths’ teeth.

The fact that the number and variety of teeth were the same in island mammoths and other mammoths doesn’t tell us anything about the overall size of the mammoths. (E) doesn’t prove that mammoths on the island were smaller.