Choose the correct adverb to fill in the blank:

The orchestra performed __________ during the prestigious music festival, impressing both the audience and the critics.

flawlessly

The correct answer is option (1) flawlessly.

An adverb is needed to describe how the orchestra performed during the prestigious music festival. "Flawlessly" is the appropriate adverb here as it describes the manner in which they performed, indicating that they performed without any mistakes or errors.