which is grammatically correct?

The first one is correct, because the verb "enhance" agrees with the singular "range."

The phrase "A wide range of syntactical structures and sophisticated word choices" is one very large noun phrase, and should be treated as a single unit. "Range" is the head of the noun phrase, the noun or pronoun around which all other parts of the phrase are arranged; therefore, it determines the plurality of the verb outside the phrase. "Range" is singular because it is a collective noun: the members of the group ("syntactical structures" and "sophisticated word choices") are considered a single unit.