The best answer is A because distinctive and prominent both similarly mean "important."
B won't work because although the paragraph does go on to tell the readers what the trunk does, the two words themselves doesn't tell the readers anything about the usage of the trunk.
C won't work because the elephants not only use their trunks when they're young, but also as they get older.
D won't work because the words doesn't tell readers what the trunk looks like. The words aren't really descriptive words.
