Presque Isle State Park, a 31,000-acre peninsula that forms a natural harbor for Erie, Pennsylvania, has five and a half miles of sandy beaches and so diverse are its forests and wetlands so that some botanists say it is one of the rarest and finest ecological preserves in the world.

A. so diverse are its forests and wetlands so

B. so diverse as its forests and wetlands are

C. with so diverse forests and wetlands

D. such a diversity of forests and wetlands

E. of such diversity its forests and wetlands

This question is based on Parallelism, Idiomatic Usage, and Structure.

The underlined portion is the second item in a list of things that Presque Isle State Park has. One is five and a half miles of sandy beaches. The other item must also be a noun phrase to maintain the parallel structure. 

In Option A, there is a redundant conjunction ‘so’. The conjunction ‘so’ has already been used at the beginning of the option. This option also lacks parallelism. The construction of the phrase ‘so---that’ is ‘so + adjective + verb + that’. However, to maintain parallelism, we need only a noun phrase. So, Option A can be eliminated.

The conjunction ‘as’ is generally paired with ‘as’ itself to imply a comparison. The phrase ‘so-as’ is used to convey a manner. Since this is not the intended meaning of the sentence and because of the lack of parallelism, Option B can be eliminated.

The presence of the preposition ‘with’ changes the structure of the underlined portion into a prepositional phrase, which is not parallel to a noun phrase. So, Option C can be eliminated.

The construction of Option E affects parallelism. The preposition at the beginning of the option makes it a prepositional phrase that is not parallel to a noun phrase. So, Option E can be eliminated.

“A diversity” is a noun. Option D therefore consists of a noun phrase that is parallel to the other noun phrase. The phrase ‘such-that’ is also idiomatically appropriate because it helps to convey emphasis. Therefore, D is the most appropriate option.