Choose the correct adverb to fill in the blank:

He spoke __________, barely audible over the noise of the crowd.

softly

The correct answer is option (1) softly.

An adverb is needed to describe how he spoke, barely audible over the noise of the crowd. "Softly" is the appropriate adverb here as it describes the manner in which he spoke, indicating that he spoke with a low volume or gentle tone.