We have singular antecedent as "Trent" so C,D,E are out.We have to use singular pronoun here...i.e; "is"
"Looking at" seems more idiomatic than "Looking to"...so IMO B is the ans...OA Pls

C. "that" has to be followed by plural "are", leaving you with choice C,D,E. Choice D and E aren't parallel, and the infinitive form is best.

OA is C

Some pointers-

1. "that" refers to communities and therefore "are" needed
2. "Looking at using" refers to visual skills rather than the logical relationship established by "looking to use"

C is parallel in every other aspect as well (through the use of infinitive form - to use, to improve)

Hence C is the correct choice

With a new park, stadium, and entertainment complex along the Delaware River, Trenton, New Jersey, is but one of a large number of communities that is looking to use its waterfront as a way for it to improve the quality of urban life and attract new businesses.
A. is looking to use its waterfront as a way for it to improve the quality of urban life and attract
B. is looking at using its waterfront to improve the quality of urban life and attract
C. are looking to use their waterfronts to improve the quality of urban life and attract
D. are looking to use its waterfront as a way of improving the quality of urban life and attracting
E. are looking at using their waterfronts as a way of improving the quality of urban life and attract

It is definitely D

"One" is similar to the indefinite SANAM pronouns (from Manhattan Gmat) which follow the case of the object (communities in this case).
Since communities is plural, we need an 'are' here
D is out because we need 'their' instead of 'its'
E changes meaning

Hope this helps

Consider Kudos for the post

+1 C

"a number of " is plural.
TO (infinitive) requires parallelism.
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The answer is C, please refer to Aristotle SC for explanation of such constructs.
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In normal terms, a non-essential relative pronoun such as ‘which’ or an essential modifier such as ‘that’ will incline to modify the noun that is just before and take an appropriate verb depending upon the number of the noun, mostly plural, ----- except when the subject ‘one’ is preceded by the definite article ‘the’.

In the context, look at the subtle difference between the two types
1. Trenton, New Jersey, is ‘but one of’ a large number of communities ‘that is looking to’ use its waterfront as a way for it to improve the quality of urban life and attract new businesses.
2. Trenton, New Jersey, is but ‘the one of’ a large number of communities ‘that is looking’ to use its waterfront as a way for it to improve the quality of urban life and attract new businesses.
Since the definite article is absent in the first case, the relative pronoun ‘that’ will modify the plural ‘communities’ and take on a plural verb ‘are looking’.
In the second case, since the definite article ‘the’ is present the focus prima-facie shifts to the article ‘the’, which restricts the number to singular. Hence, in the second case, the modifier ‘that’ in spite of its essentiality will go along taking a singular verb ‘is looking’.

In this topic, the plural verb ‘are looking’ is grammatically fitting.

That said, the choice falls among C, D and E; in the context, C is the best for using the right idiom ‘looking to use’ and for remaining parallel. D has serious SV mis-match of using ‘are looking to use its’. E uses the wrong idiom ‘looking at’ in addition to being unparallel.

There is another way to arrive at the destination without bothering about the S-V number agreement. First consider the idiom. Remove all ‘looking at’ choices. A, C and D are left Then dump D for the blatant S-V mismatch. A is wordy and imprecise. C is forthright and crisp. Done?
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Thanks a lot daagh. +1 kudos for u.

was thinking it would be A. but read the explanations and it makes sense. A number of is plural so must go with are. BTW one doesnot figure in the SANAM pronouns of Manhattan. for indefinite pronouns ending with 'one, body or thing' eg. somebody are almost always singular.