**Core Concept**
The effective temperature of the 'comfort zone' is a measure of the combined effects of temperature, humidity, and air movement on human comfort. It is a concept used in environmental physiology to assess the thermal comfort of individuals in various environments.

**Why the Correct Answer is Right**
The comfort zone is typically defined as an effective temperature range of 20-24°C (68-75°F) with a relative humidity of 50-60%. This range allows for optimal heat loss and helps to maintain a stable body temperature. The effective temperature is calculated using the following formula: ET = (t + 0.5 × (rh / 100) + (w / 17.8)) × 0.55 + (w / 355), where t is the dry-bulb temperature, rh is the relative humidity, and w is the wind speed. The comfort zone is essential for preventing heat-related illnesses, such as heat exhaustion and heat stroke.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is too low, and such a temperature would be uncomfortable for most people.
* **Option B:** This option is incorrect because it is too high, and temperatures above 27°C (80.6°F) can lead to heat-related illnesses.
* **Option C:** This option is incorrect because it is not a valid effective temperature range for the comfort zone.

**Clinical Pearl / High-Yield Fact**
The comfort zone is critical in preventing heat-related illnesses, especially in high-risk populations such as the elderly, young children, and individuals with pre-existing medical conditions.

**Correct Answer:** 23.