Final answer:
In the sentence, 'beautifully' is an adverb since it describes the manner in which 'they' perform the action of the verb 'sing'.
Step-by-step explanation:
The word beautifully in the sentence "They sing beautifully during the entire winter concert." is an adverb. This conclusion can be made because beautifully describes how they sing, which is the action being performed in the sentence. Hence, it's modifying the verb 'sing' rather than a noun or pronoun, which would make it an adjective. Adverbs often describe or modify the action of a verb and can answer questions such as 'how?', 'when?', 'where?', and 'to what extent?'. In this example, 'beautifully' is answering the question 'How do they sing?'.