**Core Concept**
The human body loses heat through various mechanisms, including radiation, conduction, convection, and evaporation. In a resting individual, the primary mode of heat loss depends on the environmental conditions, such as temperature and humidity.

**Why the Correct Answer is Right**
In a humid environment with a high relative humidity of 80%, the rate of evaporation from the skin is significantly reduced. As a result, the body's primary means of heat loss, which is evaporation of sweat, is impaired. In this scenario, the body relies on other mechanisms to lose heat. Convection is the most effective means of heat loss when the air temperature is close to the body temperature, as it involves the transfer of heat from the body surface to the surrounding air. Since the air temperature is 21°C, which is close to the normal body temperature, convection becomes the primary mode of heat loss.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because radiation is a significant mode of heat loss, but it is not the greatest amount of heat loss in this scenario. Radiation occurs regardless of the environment and is not affected by humidity or temperature.
* **Option B:** This option is incorrect because conduction is not a significant mode of heat loss in this scenario. Conduction requires direct contact between the body and a cooler surface, which is not present in this situation.
* **Option D:** This option is incorrect because respiration is not a significant mode of heat loss in this scenario. While the body does lose heat through respiration, it is not the greatest amount of heat loss in this environment.

**Clinical Pearl / High-Yield Fact**
In hot and humid environments, the body's ability to cool itself through sweating is impaired. This can lead to heat-related illnesses, such as heat exhaustion and heat stroke. It is essential to take precautions to prevent heat-related illnesses in such environments.

**Correct Answer: C. Convection. Heat loss through convection is the greatest amount.**