Correct option is A
Option (C) contains the error.
Explanation: The word "beautiful" is an adjective, but it should be "beautifully", an adverb, because it modifies the verb "sings". Adverbs are used to describe actions.
Grammatical rule used: Adjectives describe nouns, while adverbs modify verbs, adjectives, or other adverbs.
Correct sentence: She sings the song beautifully in the party.
Example: He speaks fluently, not fluent.