## Core Concept
The question assesses knowledge of methods used to test the efficacy of disinfectants. Disinfectants are chemical agents designed to inactivate or destroy microorganisms on inert surfaces. Various tests are employed to evaluate their effectiveness, including the **Rideal-Walker test**, **Chick-Martin test**, and **Kelsey-Soper test**.

## Why the Correct Answer is Right
The **Rideal-Walker test**, **Chick-Martin test**, and **Kelsey-Soper test** are established methods for evaluating the efficacy of disinfectants.
- The **Rideal-Walker test** involves comparing the disinfectant's effectiveness against a standard test organism (usually Salmonella typhi) on a surface.
- The **Chick-Martin test** modifies the Rideal-Walker test by incorporating organic matter to simulate real-world conditions.
- The **Kelsey-Soper test** evaluates a disinfectant's ability to kill bacteria in the presence of organic matter over a specified contact time.

## Why Each Wrong Option is Incorrect
- **Option A: Rideal-Walker test** is indeed a test for disinfectants, historically significant in assessing phenolic disinfectants.
- **Option B: Chick-Martin test** is also a valid test, enhancing the Rideal-Walker test by adding organic matter.
- **Option C: Kelsey-Soper test** is another recognized method.

## Why D is Correct
- **Option D: Schick test** is not a test for disinfectants; it's actually related to assessing immunity to diphtheria.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the efficacy of disinfectants can be influenced by factors such as **organic matter**, **contact time**, and **concentration of the disinfectant**. Understanding these factors and the standard tests for disinfectant efficacy is crucial for ensuring appropriate infection control practices.

## Correct Answer: D. Schick test