Unlike most warbler species, the male and female blue-winged wa

Excellent question! You're right that because the subject of the first clause, "those," is plural, it should logically refer to the plural subject of the subsequent clause, "the male and female blue-winged warblers." But if we substitute the referent in place of "those," we get, "Unlike the male and female blue-winged warblers in most warbler species, the male and female blue-winged warblers are very difficult to distinguish.

How can male and female blue-winged warblers be unlike themselves??? That's just nutty. So (C) is out.

Here's (E) again:

• Blue-winged warblers are unlike most species of warbler in that it is very difficult to tell the male and female apart.

While "in that" sounds a little goofy to my ear, we never want to eliminate answer choices simply because they sound a little off. Otherwise the question writer can seduce us into selecting the wrong answer every single time by having the OA sound a funny.

In this case, "in that" is used to introduce a supporting example. How are blue-winged warblers unlike most species of warbler? In that it's difficult to tell the sexes apart. Makes sense to me.

Most importantly, don't waste brain space obsessing over esoteric constructions, such as "in that." Instead, just know that on difficult SC questions, the correct answer will sometimes sound a little strange to your ear, and this is intentional. It's why we want to use logic, meaning, and grammar to eliminate incorrect options, rather than relying on our ear.

I hope that helps!