What is the meaning of 'ad lib.' in music?

The term 'ad lib.' in music is derived from the Latin phrase "ad libitum," which literally means "at one's pleasure." This indicates that the performer has the flexibility to interpret the music in a way that feels natural and personal to them, rather than strictly adhering to the written notes or tempo. This can involve improvisation or making spontaneous changes to the rhythm, melody, or dynamics of the piece.

The other options represent specific directives in music, such as how a piece should be played or interpreted, but they do not convey the concept of performer autonomy that 'ad lib.' signifies. For instance, "in the manner of the composer" implies a strict adherence to the composer's style, while "gradually slow down" specifically describes a change in tempo. "With energy" relates to performance dynamics rather than the freedom to improvise, which is the essence of 'ad lib.' Thus, the correct understanding of 'ad lib.' highlights the performer's liberty to express themselves within the framework of the piece.