Final answer:
In music, "rubato" refers to expressive timing variations made by a performer, affecting the rhythm of the piece to enhance its emotional expression.
Step-by-step explanation:
The term "rubato" in music refers to a flexible tempo where the performer makes slight, expressive changes to the speed of the piece. This technique primarily affects the rhythm of the music, allowing for a more expressive performance. As such, the correct answer to the student's question is option d. rhythm. Rubato lets the performer speed up or slow down at certain points, which can emphasize the emotional content of a piece without altering the overall structure or written notes.