What aspect of singing does "ad libitum" allow the performer to adjust?

"Ad libitum" is a term from music that allows performers to take liberties with their performance, particularly in terms of interpretation. By choosing "C", it highlights melodic variations as the aspect that ad libitum most directly influences. This means that a singer can alter the melody, adding embellishments or improvisations, making the performance more expressive and personal.

In this context, while the other options relate to elements of music performance, they do not encapsulate the core essence of ad libitum as effectively as melody variations do. Speed, dynamics, and rhythm are more about the structural aspects of the piece, and while they can be influenced creatively, they are not as inherently linked to the improvisational freedom that ad libitum provides. Thus, the emphasis on the ability to adjust melody aligns perfectly with the spirit of ad libitum.