The answer is D. Because of the wording of the sentence, the base word enthusiastic must be used as an adverb, not an adjective.
The phrase that best completes the sentence is d. the most enthusiastically. The blank in the sentence requires an adverb to describe how Anthony celebrated. Since we are comparing Anthony's level of celebration with all the fans, we need the superlative form of the adverb, which is 'the most enthusiastically'.
The other options either do not use the superlative form or incorrectly use an adjective where an adverb is required.
The correct phrase to complete the sentence is 'the most enthusiastically', as it is the superlative adverb needed to describe how well Anthony celebrated compared to all the fans. Using an adverb is essential here because we are looking at the manner of celebration. Thus, the best option is D. the most enthusiastically.
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