**Core Concept**
The question tests the knowledge of chemical testing methods for detecting free chlorine in water, specifically the orthotoluidine test. **Orthotoluidine** is a reagent used to detect the presence of **free chlorine** in water samples. This test is based on a chemical reaction that produces a color change.

**Why the Correct Answer is Right**
When **orthotoluidine** reagent is added to water containing **free chlorine**, it reacts with the chlorine to produce a yellow-colored compound. This color change is indicative of the presence of **free chlorine** in the water sample. The intensity of the yellow color is directly proportional to the concentration of **free chlorine** present.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the reaction between orthotoluidine and free chlorine does not produce a colorless solution.
**Option B:** This option is incorrect because the color produced is not green.
**Option D:** This option is incorrect because the color produced is not blue.

**Clinical Pearl / High-Yield Fact**
The orthotoluidine test is a simple and effective method for detecting **free chlorine** in water, which is essential for ensuring the safety of drinking water.

**Correct Answer:** D. Yellow