Option A is the answer.
Female sparrows and immatures are somewhat nondescript, while the typical adult male sparrow is easily recognized by its markings; a black bib, a grey cap, and white lines trailing down from the mouth.
This sentence compares and contrasts the observable characteristics of the female and male sparrow. It can be inferred by the contrasting phrases in Option A.
In Option B, “difficult to spot, better camouflaged” both mean the same thing, and there is no contrast.
As Option C states, there is no data to support the fact that female sparrows are “considered to be endangered”, so this is INCORRECT.
Option D would mean that female sparrows are “comparatively small”. There is no evidence, within the given sentence, to support this statement about size. The only characteristics mentioned are markings (a black bib, a grey cap, and white lines trailing down from the mouth).