A) Unlike most warbler species, the male and female blue-winged warbler are very difficult to tell apart.
Wrong comparison.
It compares warbler species to male and female blue-winged warbler.
But warbler species must be compared directly to blue-winged warbler.
A is wrong.
(B) Unlike most warbler species, the gender of the blue-winged warbler is very difficult to distinguish.
Wrong comparison.
Here, warbler species are compared to the gender.
B is wrong.
(C) Unlike those in most warbler species, the male and female blue-winged warblers are very difficult to distinguish.
Wrong comparison.
'those' does not have a referant. We understand 'those' refers to genders, but the word 'genders' is not present anywhere in the sentence. So, 'those' does not have an antecedant.
Instead, the correct answer should be (if we want 'those' to refer to genders) :
Unlike those(genders) in most warbler species, the genders in blue-winged...
C is wrong.
(D) It is very difficult, unlike in most warbler species, to tell the male and female blue-winged warbler apart.
Wrong comparison.
Here, again, warbler species are incorrectly compared to the male and female blue-winged warblers.
D is wrong.
(E) Blue-winged warblers are unlike most species of warbler in that it is very difficult to tell the male and female apart.
Correct comparison.
Blue-wingled warblers are correctly compared to warbler species.
E is correct.
Because v/s. In That: When GMAT puts ‘because’ and ‘in that’ in a sentence, more often than not, ‘in that’ would be correct. ‘In that qualifies’ the previous sentence, while ‘because’ is just used to show a simple causal relationship.