Final answer:
Frequency is to pitch as amplitude is to loudness, with loudness being measured in phons. The correct option is C.
Step-by-step explanation:
In the context of sound, frequency is related to the perception of pitch, while amplitude is related to the perception of loudness. Loudness is measured in units called phons. Therefore, if frequency is to pitch, then amplitude is to loudness, making the correct answer to the student's question (C) pitch; loudness.
Understanding these relationships is critical in the physics of sound and in various applications such as music and audio engineering where both pitch and loudness play key roles in the creation and perception of sound.