**Core Concept**
The question is testing the physiological response to changes in sound pitch, specifically how the auditory system adapts to increased pitch. This involves the perception of sound frequencies and the neural processing of auditory signals.

**Why the Correct Answer is Right**
As the pitch of a sound increases, the frequency of the sound wave also increases. The auditory system responds to this change by increasing the firing rate of auditory nerve fibers, which are responsible for transmitting sound information from the cochlea to the brain. This increased firing rate is due to the stiffer basilar membrane in the cochlea, which allows for faster vibration and transmission of higher frequency sounds. The brain interprets this increased firing rate as an increase in sound pitch.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the response to changes in sound pitch does not involve the contraction of muscles in the middle ear.
**Option B:** This option is incorrect because the response to changes in sound pitch does not involve the production of more sound waves by the external ear.
**Option C:** This option is incorrect because the response to changes in sound pitch does not involve the activation of more neurons in the auditory cortex.

**Clinical Pearl / High-Yield Fact**
The perception of sound pitch is closely related to the frequency of the sound wave, and the auditory system is able to detect subtle changes in frequency through the use of different types of hair cells in the cochlea.

**Correct Answer: D. Increased firing rate of auditory nerve fibers.**