**Core Concept**
Turbidity is a measure of the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. In medical and laboratory settings, turbidity is an essential parameter in assessing the quality and purity of various fluids, including water, blood, and medications.

**Why the Correct Answer is Right**
The correct unit of turbidity is the Nephelometric Turbidity Unit (NTU). This unit is used to express the concentration of suspended particles in a fluid. It is calculated by measuring the amount of light scattered by particles in a fluid when a light beam passes through it. The NTU is a standardized unit that allows for accurate comparison of turbidity levels across different fluids and laboratory settings. The nephelometric method involves measuring the light scattered at a 90-degree angle to the incident light beam, which is then used to calculate the turbidity.

**Why Each Wrong Option is Incorrect**
**Option A:** Turbidity can be expressed in other units such as Jackson Turbidity Units (JTU), but NTU is the most commonly used and standardized unit.
**Option B:** Nephelometric Turbidity Units (NTU) is the correct answer, not Nephelometric Units (NU).
**Option D:** The correct answer is NTU, not Formazin Turbidity Units (FTU), which is also a unit of turbidity, but not the most commonly used one.

**Clinical Pearl / High-Yield Fact**
It's essential to note that turbidity is an important parameter in assessing the quality of water and other fluids, especially in medical settings where accurate measurement of fluid purity is crucial. A high turbidity level can indicate the presence of suspended particles, which may lead to adverse reactions or complications in patients.

**Correct Answer: C. Nephelometric Turbidity Unit (NTU)**