Official Explanation:
This sentence begins by describing the versatility of goats: they are just as comfortable in treacherous mountain landscapes as on more pastoral grounds, leading to the more general claim that goats are more adaptable than sheep.
(A) Craggiest, a superlative that compares the given landscapes to others, does not make sense in parallel to treacherous, an adjective that does not. The portion of this choice preceding the semicolon is a fragment, not a complete sentence as required.
(B) In the construction as comfortable scrambling as to perch, scrambling and to perch should be parallel but are not. In addition, this sentence is a run-on: if the intervening modifier (as comfortable…knolls) is eliminated, the resulting sentence reads: The goat it is more adaptable. A sentence cannot have two subjects without a conjunction in between.
(C) Craggiest, a superlative that compares the given landscapes to others, does not make sense in parallel to treacherous, an adjective that does not. In addition, the plural noun goats does not match with the singular pronoun its. The sheep cannot be its own cousin, so the pronoun its has no acceptable antecedent.
(D) The plural noun goats does not match with the singular pronoun its. The sheep cannot be its own cousin, so the pronoun its has no acceptable antecedent.
(E) CORRECT. The singular noun the goat serves as a proper antecedent for the pronoun its. The superlatives craggiest and most treacherous, both of which compare the given landscapes to others, make sense in parallel. Scrambling and perching are also parallel.