GMAT Sentence Correction (SC) | Re: Most state governors now have the power

Option B changes the meaning of the sentence. "not yet available" implies that such power would probably be available to the U.S. President in the future.

Option C is ungrammatical --

"line item veto, which the US President has no such power" does not make any sense.

Option D is a run-on sentence.

Option E is also a run-on sentence. The pronoun "that" is also problematic.
Note that we cannot use demonstrative pronouns such as "this/that/these/those" independently.
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