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Sentence Correction: Sentence vs. Clause vs. Fragment

Ecuador's extraordinary topographical diversity comprises snow-capped peaks reaching an altitude of more than 20,000 feet and rain-forest ravines descending steeply toward the Amazon basin, supporting an equally astounding diversity of wildlife species.

Incorrect.

This answer choice is stylistically flawed due to ambiguity. It is unclear what the phrase supporting an equally astounding diversity of wildlife species refers to. While it should logically refer to Ecuador's extraordinary topographical diversity, it may also refer to snow-capped peaks, rain-forest ravines, or the Amazon basin.

Great!

This answer choice solves the ambiguity of the original sentence by changing the main verb comprises to the unconjugated verb comprising (and thus turning almost the entire underlined section into a modifier) and by changing the unconjugated verb supporting to the conjugated verb supports (making it the main verb).

If we look at the sentence without the new modifier, it makes sense:

Ecuador's extraordinary topographical diversity , ... , supports an equally astounding diversity of wildlife species.

Incorrect.

This answer choice is illogical. In this sentence, the phrase and descends steeply towards the Amazon basin is a part of a parallelism of verbs (comprises and descends) whose subject is Ecuador's extraordinary topographical diversity. However, it is illogical to say that diversity descends into the Amazon basin.

Incorrect.

While this answer choice is grammatically correct, it is ambiguous. In this sentence, the phrase and descending steeply towards the Amazon basin may be a part of a parallelism with either comprising (comprising ... and descending) or reaching (reaching ... and descending). 

Furthermore, both options are invalid: in the former descending illogically refers to diversity and in the latter descending refers to snow-capped peaks, whereas in the original meaning it refers to the rain-forest ravines.

Incorrect.

This answer choice is grammatically incorrect. In this sentence there are two conjugated verbs in the main clause: comprises and is supporting. A clause may only have one conjugated verb (the main verb) unless a connector (and) is used to connect them.

To see how this happened, lets compact the object of comprises (snow-capped peaks...) into 'X':

Ecuador's extraordinary topographical diversity comprises X, is supporting an equally astounding diversity of wildlife species.

diversity comprises snow-capped peaks reaching an altitude of more than 20,000 feet and rain-forest ravines descending steeply toward the Amazon basin, supporting
diversity, comprising snow-capped peaks reaching an altitude of more than 20,000 feet and rain-forest ravines that descend steeply toward the Amazon basin, supports
diversity comprises snow-capped peaks that reach an altitude of more than 20,000 feet and rain-forest ravines and descends steeply toward the Amazon basin, supporting
diversity, comprising snow-capped peaks reaching an altitude of more than 20,000 feet and rain-forest ravines and descending steeply toward the Amazon basin, supports
diversity comprises snow-capped peaks reaching an altitude of more than 20,000 feet and rain-forest ravines descending steeply toward the Amazon basin, is supporting