## **Core Concept**
The question tests the understanding of a chemical test for detecting free chlorine in water using orthotolidine (also known as o-tolidine) reagent. Free chlorine in water is a critical parameter, especially in water treatment and disinfection processes.

## **Why the Correct Answer is Right**
When orthotolidine reagent is added to water containing free chlorine, the chlorine oxidizes the orthotolidine, resulting in the formation of a yellow-colored compound. This reaction is a basis for a simple and rapid test for the presence of free chlorine. The yellow color produced is directly proportional to the concentration of free chlorine present. Therefore, the correct answer is associated with the production of a yellow color.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not correspond to the correct color produced in the reaction with free chlorine.
- **Option B:** Similarly, this option is incorrect as it does not represent the yellow coloration expected.
- **Option D:** This option is also incorrect for the same reason; it does not match the expected yellow color.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the presence of free chlorine in drinking water is crucial for disinfection and killing pathogens. However, excessive levels can be harmful. The orthotolidine test, although simple, is a useful tool for quickly assessing free chlorine levels. A classic clinical correlation is ensuring water safety in public health settings.

## **Correct Answer:** . Yellow.