**Core Concept**
The Rideal-Walker coefficient is a measure used to express the effectiveness of a disinfectant against a particular type of microbial contamination. It is calculated by comparing the concentration of the disinfectant required to kill a specific number of microorganisms in a given time to the concentration of phenol required to achieve the same effect. This coefficient provides a quantitative assessment of a disinfectant's potency and is widely used in healthcare settings to evaluate the efficacy of disinfectants against various pathogens.

**Why the Correct Answer is Right**
The Rideal-Walker coefficient is calculated using the following formula: C × t (concentration × time) required for the disinfectant to achieve a certain level of microbial kill, divided by the C × t required for phenol to achieve the same level of kill. This coefficient helps to standardize the comparison of disinfectants and provides a more accurate representation of their effectiveness. The Rideal-Walker coefficient is particularly useful in evaluating the effectiveness of disinfectants against bacteria, viruses, and fungi.

**Why Each Wrong Option is Incorrect**
**Option A:** The Dettre-Hamilton test is a different method used to evaluate the effectiveness of surfactants, not disinfectants.
**Option B:** The phenol coefficient is a related concept, but it is not the same as the Rideal-Walker coefficient, which is a more comprehensive measure of a disinfectant's effectiveness.
**Option C:** The Sporicidal activity of a disinfectant is an important aspect of its effectiveness, but it is not directly related to the Rideal-Walker coefficient.

**Clinical Pearl / High-Yield Fact**
The Rideal-Walker coefficient is an important consideration when selecting a disinfectant for use in healthcare settings, particularly in areas where pathogen control is critical, such as operating rooms and intensive care units.

**Correct Answer: C. Disinfectant effectiveness**