I would stick with A as it used correctly
Unlike X, Y
And I don't like "but" part in B

To maintain ||ism 'Unlike crested wheatgrass' needs to be ||el with 'native North American grasses'. This leaves us only A and B.

In B, I think 'their' is very confusing, also 'but without growing' is very awkward.

gmat_perfect wrote:Unlike crested wheatgrass, an alien species from Siberia that forms only shallow roots and produces tall above-ground shoots, native North American grasses develop an extensive root system, allowing for greater enrichment of the soil, but do not grow to be very tall.

(A) native North American grasses develop an extensive root system, allowing for greater enrichment of the soil, but do not grow to be very
(B) native North American grasses develop an extensive root system, which allows for their greater enrichment of the soil, but without growing to be that
(C) the root system of native North American grasses is extensive, allowing for greater enrichment of the soil, but they do not grow to be very
(D) the root system of native North American grasses is more extensive, allowing for the greater enrichment of the soil, but these grasses do not grow to be very
(E) the extensiveness of the root system of native North American grasses allows for their greater enrichment of the soil, but they do not grow to be that

[spoiler]OA: A[/spoiler]

Final solution at one place:

Important: The purpose of this post (and all the other posts by me) is to give a complete solution to all GMAT-Prep Verbal questions at one place. Sometimes students have to wade through dozens of posts to get to the final answer. My posts will give one complete and crisp solution required to arrive at the correct answer by eliminating the wrong one. Some of the content in these posts may have been taken from various other sources (discussion forums).

Unlike crested wheatgrass, an alien species from Siberia that forms only shallow roots and produces tall above-ground shoots, native North American grasses develop an extensive root system, allowing for greater enrichment of the soil, but do not grow to be very tall.
(A) native North American grasses develop an extensive root system, allowing for greater enrichment of the soil, but do not grow to be very
(B) native North American grasses develop an extensive root system, which allows for their greater enrichment of the soil, but without growing to be that
(C) the root system of native North American grasses is extensive, allowing for greater enrichment of the soil, but they do not grow to be very
(D) the root system of native North American grasses is more extensive, allowing for the greater enrichment of the soil, but these grasses do not grow to be very
(E) the extensiveness of the root system of native North American grasses allows for their greater enrichment of the soil, but they do not grow to be that

Unlike crested wheatgrass (X) ... native North American grasses (Y) ... correct parallelism and comparison. This eliminates C, D, and E.

In A, "allowing for greater enrichment of the soil" modifies the entire clause of "native North American grasses develop an extensive root system" when you have an -ING MODIFIER PRECEDED BY A COMMA, it's an adverbial modifier that modifies the entire preceding clause.

Also, 'that tall' is not correct in B. This is acceptable in colloquial English but definitely not in formal written English. This eliminates B.

If you say, John is 6 feet tall but Peter is not 'that' tall. In formal English, this sentence is considered incorrect because 'tall' is an adjective; to modify an adjective, we need an adverb such as 'so' or 'very' etc. 'That' is not an adverb.

A: Correct