**Core Concept**
Carbolic acid poisoning, also known as phenol poisoning, is a toxic condition caused by the ingestion of phenol, a corrosive and toxic substance. The colour change of urine in carbolic acid poisoning is an important clinical indicator.

**Why the Correct Answer is Right**
In carbolic acid poisoning, the urine turns **cherry red** due to the oxidation of phenol to quinones, which react with the chromogens in the urine to produce this characteristic colour. This colour change is a diagnostic clue in cases of suspected carbolic acid poisoning. The oxidation of phenol is facilitated by the presence of oxygen, which is abundant in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the urine does not turn green in carbolic acid poisoning. The metabolism of phenol does not produce green pigments.
**Option B:** This option is incorrect because the urine does not turn blue in carbolic acid poisoning. The oxidation of phenol does not produce blue pigments.
**Option C:** This option is incorrect because the urine does not turn black in carbolic acid poisoning. The metabolism of phenol does not produce melanin-like substances.

**Clinical Pearl / High-Yield Fact**
The colour change of urine in carbolic acid poisoning is a classic example of a **chemical alteration** of a bodily fluid, which is a common exam question in medical toxicology.

**Correct Answer:** C.