**Core Concept**
The pure tone audiogram is a graphical representation of an individual's hearing sensitivity across different frequencies. It is a crucial tool in the diagnosis and assessment of hearing loss. The audiogram plots the threshold of hearing in decibels (dB) against the frequency of the sound in Hertz (Hz).

**Why the Correct Answer is Right**
The symbol X is used to mark the threshold of hearing at a particular frequency on the pure tone audiogram. This indicates the minimum intensity of sound that an individual can detect at that specific frequency. The X marks the point where the individual's hearing threshold intersects with the sound pressure level (SPL) scale on the audiogram. In other words, the X represents the softest sound the individual can hear at a given frequency.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the symbol "O" is typically used to mark the bone conduction threshold, not the air conduction threshold.
**Option B:** This option is not correct because the symbol "O" is used for bone conduction, and the question specifically asks about the air conduction threshold marked by the symbol X.
**Option C:** This option is incorrect because the symbol "O" is used for bone conduction, and the question specifically asks about the air conduction threshold marked by the symbol X.

**Clinical Pearl / High-Yield Fact**
When interpreting pure tone audiograms, it's essential to note that the X marks the air conduction threshold, while the O marks the bone conduction threshold. This distinction is crucial in differentiating between conductive and sensorineural hearing losses.

**Correct Answer: C. The symbol X is used to mark the air conduction threshold on a pure tone audiogram.**