During the brief ___ in the music, someone's ringing cell phone split the air.

The correct choice is 'hiatus.' A hiatus refers to a pause or break in a sequence or activity, which, in this context, aptly describes a gap or interruption in the music. This interruption creates a moment where the ringing cell phone becomes notably intrusive, thus emphasizing the suddenness of the intruding noise in contrast to the preceding musical ambiance.

On the other hand, while 'intro' refers to the beginning section of a piece of music, it doesn't capture the sense of a pause. 'Bridge' pertains to a transitional section in a song that connects different parts, again lacking the notion of a break. 'Measure' relates to a specific unit of time in music, which does not fit the context of a brief interruption. Overall, 'hiatus' effectively conveys the idea of a disruption in the flow of music, making it the most suitable choice.