**Core Concept**
The threshold of hearing is the minimum sound intensity that an individual can detect, and it is a critical parameter in audiology. The unit of intensity used to measure this threshold is a fundamental concept in understanding hearing sensitivity.

**Why the Correct Answer is Right**
The unit of intensity used to measure the threshold of hearing in an audiogram is the **decibel (dB)**. This is because decibels are a logarithmic scale that accurately represent the range of sound intensities that the human ear can detect. The decibel scale is also proportional to the perceived loudness of a sound, making it a suitable unit for measuring hearing thresholds. The sound pressure level (SPL) in decibels is calculated using the formula: SPL (dB) = 20 log10 (P/P0), where P is the sound pressure and P0 is the reference pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a unit of intensity. While sound pressure is related to intensity, it is not the unit used to measure the threshold of hearing.
* **Option B:** This option is incorrect because it is a unit of time, not intensity. The second is a fundamental unit of time in the International System of Units (SI).
* **Option C:** This option is incorrect because it is a unit of frequency, not intensity. The hertz (Hz) is a unit used to measure the frequency of sound waves.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the decibel scale is logarithmic, which means that small increments in decibels correspond to much larger increases in sound intensity. For example, a 10 dB increase in sound intensity corresponds to a tenfold increase in sound pressure.

**Correct Answer:** C. decibel (dB)