Hailed as a key discovery in the science of evolution,
the fossils of a large scaly creature resembling both a fish and a land-animal provide evidence of a possible link in the evolutionary chain from water-based to land-based organisms.
(A) the fossils of a large scaly creature resembling both a fish and a land-animal provide evidence of -> fossils is right modifier for "Hailed as a key discovery....". There is no such error. Let's keep it.
(B) a large scaly creature resembling both a fish and a land-animal provides fossils that are evidence -> Modifier error. "a large scaly creature" is not "Hailed as a key discovery...". Incorrect.
(C) a large scaly creature, whose fossils resemble both a fish and a land-animal, provides evidence of -> Modifier error. Same as B. Incorrect.
(D) the fossils of a large scaly creature, which resembles both a fish and a land-animal, provides evidence of -> Subject verb agreement error. "the fossils" needs plural verb, but we have "provides" singular verb. Incorrect.
(E) the fossils of a large scaly creature resemble both a fish and a land-animal and provide evidence of -> fossils is not right modifier for "both a fish and a land-animal....." Incorrect.
So, I think A.