Unlike most warbler species, the male and female blue-winged warbler are very difficult to tell apart.Option Elimination -
(A) Unlike most warbler species, the male and female blue-winged warbler are very difficult to tell apart. - "Most warbler species" are compared with "the male and female": the subject for the following clause. Wrong.
(B) Unlike most warbler species, the gender of the blue-winged warbler is very difficult to distinguish. - "Most warbler species" are compared with "the gender." Wrong.
(C) Unlike those in most warbler species, the male and female blue-winged warblers are very difficult to distinguish. - "Those" can refer to "the male and female," "blue-winged warblers," or "the male and female blue-winged warblers." The last two don't make sense. Wrong.
(D) It is very difficult, unlike in most warbler species, to tell the male and female blue-winged warbler apart. - we need "warblers" and not singular for "male and female." Moreover, like/unlike is usually followed by a noun or noun phrase. There is no rule, but it's a general preference, as prepositional phrases can be adverbial.
(E) Blue-winged warblers are unlike most species of warbler in that it is very difficult to tell the male and female apart. - There is one trap here. I know most of us look for "other" here. It is possible that the "Blue-winged warblers" are outside that set of "most species of warbler," so the "other" thought it good to have but not a must. But had it been "all species," we would 100% need "other."
"in that" introduces the intrinsic property. E.g., Gold is considered noble in that it doesn't react with other elements. Another adverbial could be "because," but because is normally used for cause and effect.