What does a group of notes and rests separated by vertical lines illustrate?

A group of notes and rests that are separated by vertical lines illustrates a measure. In musical notation, a measure serves as a basic unit of time, organizing beats and providing a way to structure the rhythm within a piece of music. The vertical lines, known as bar lines, help to define the beginning and end of each measure, allowing musicians to keep track of the rhythm and maintain a consistent pace throughout the composition. This organization is essential for proper timing and interpretation of the music, as it divides the musical staff into manageable segments.

In contrast, a time signature indicates how many beats are in a measure and what note value is equivalent to one beat, but it does not visually represent notes or rests directly. A key signature indicates the key of the piece by designating which notes are sharp or flat, impacting the tonality of the music, but also does not visually separate notes and rests. A clef symbol defines the pitch range for the notes on a staff but serves a different purpose in notation, as it does not relate to the grouping of rhythms within measures. Thus, the measure is the correct answer as it directly involves the organization of notes and rests in music.