**Core Concept**
The pure tone audiogram is a graphical representation of hearing threshold levels in different frequency ranges, used to assess hearing loss and identify its type. The audiogram plots sound intensity (in decibels) against frequency (in Hertz), helping clinicians diagnose various types of hearing impairment.

**Why the Correct Answer is Right**
The symbol 'X' in a pure tone audiogram is used to mark the bone conduction threshold, specifically the level at which a person can no longer hear a sound through the bone (osseous transmission). This is in contrast to air conduction (AC) thresholds, which are marked with an open circle. The difference between AC and bone conduction thresholds helps determine the type and location of hearing loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the symbol 'O' is typically used to mark the air conduction threshold, not the bone conduction threshold.
**Option B:** Incorrect because the symbol 'X' is indeed used in the context of pure tone audiograms, but not to mark the air conduction threshold.
**Option C:** Incorrect because there is no universally accepted symbol 'C' in pure tone audiograms to mark a specific threshold.

**Clinical Pearl / High-Yield Fact**
When interpreting pure tone audiograms, remember that the difference between air conduction (AC) and bone conduction (BC) thresholds can help identify conductive hearing loss (if AC thresholds are lower than BC thresholds) or sensorineural hearing loss (if AC thresholds are higher than BC thresholds).

**Correct Answer: B.**