## Core Concept
Horrock's Apparatus is used for the determination of the chlorine demand of water. The indicator solution in Horrock's Apparatus is crucial for determining the endpoint of the titration.

## Why the Correct Answer is Right
The correct answer, **Eosin**, is a type of acid dye that acts as an indicator in Horrock's Apparatus. In the context of Horrock's Apparatus, eosin is used because it changes color at the endpoint of the titration, indicating that the chlorine demand has been met. This is typically relevant in water treatment processes.

## Why Each Wrong Option is Incorrect
* **Option A:** Bromothymol blue is a pH indicator used in various chemical and biological assays but is not specifically used in Horrock's Apparatus.
* **Option B:** Methylene blue is a dye used in biology and medicine for staining and as an antidote for methemoglobinemia but is not the indicator in Horrock's Apparatus.
* **Option D:** Ferric alum (or ferric ammonium sulfate) is used in water treatment and as a mordant in dyeing but is not the indicator solution in Horrock's Apparatus.

## Clinical Pearl / High-Yield Fact
A key point to remember is that eosin is utilized as an indicator in specific titration processes, including Horrock's Apparatus for assessing chlorine demand in water. Understanding the role of eosin and other indicators is essential for accurate laboratory and clinical measurements.

## Correct Answer Line
**Correct Answer: C. Eosin**