Rebecca tells Tom that he is singing "off pitch." Rebecca is referring to which physical property of sound?

When Rebecca tells Tom that he is singing "off pitch," she is referring to the frequency of the sound. Frequency determines the pitch of a sound; higher frequencies correspond to higher pitches and lower frequencies correspond to lower pitches. When someone is "off pitch," it typically means that the frequency of their singing does not match the intended musical note.

Loudness, on the other hand, relates to the amplitude of the sound wave and describes how intense or soft a sound is, rather than its pitch. Amplitude refers specifically to the height of the sound wave and affects the loudness but does not influence the perception of pitch. Speed of sound relates to how fast sound travels through a medium, but it doesn't relate to the quality of the pitch itself. Therefore, frequency is the correct choice connected to pitching in music and vocal performance.