**Core Concept**
The Horrock's apparatus is a device used to determine the chlorine demand of water, which measures the amount of chlorine required to oxidize the organic matter present in the water. This process is essential in water treatment to ensure the water is safe for consumption.

**Why the Correct Answer is Right**
The reagent used in Horrock's apparatus is orthotolidine (OT). When chlorine reacts with orthotolidine, it forms a yellow-colored compound, which is directly proportional to the amount of chlorine present. This reaction allows for the quantitative determination of chlorine demand. The orthotolidine-chlorine reaction is a specific example of an oxidative-reductive reaction, where the chlorine molecule donates an electron to the orthotolidine, resulting in the formation of the colored compound.

**Why Each Wrong Option is Incorrect**
**Option A:** **No information provided**
We cannot evaluate the correctness of Option A without knowing what it is.

**Option B:** **No information provided**
Similarly, we cannot assess the accuracy of Option B without knowing its contents.

**Option C:** **No information provided**
Again, without the contents of Option C, we cannot evaluate its correctness.

**Clinical Pearl / High-Yield Fact**
One important consideration when using the Horrock's apparatus is to ensure the orthotolidine is fresh and not contaminated, as this can affect the accuracy of the results. Regular calibration of the apparatus is also essential to maintain its reliability.

**Correct Answer: D. Orthotolidine**