**Core Concept**
The heat stress index (HIS) is a measure used to assess the risk of heat-related illnesses in individuals exposed to high temperatures. It takes into account both the air temperature and the humidity level, providing a more accurate prediction of the body's ability to dissipate heat.

**Why the Correct Answer is Right**
In severe heat strain, the body's ability to lose heat through sweating is impaired due to the high humidity levels. This leads to a significant increase in the body's core temperature, which can cause heat-related illnesses such as heat exhaustion and heat stroke. The heat stress index is calculated using the following formula: HIS = (air temperature - dew point temperature) / 5. The resulting value is then used to categorize the risk of heat-related illnesses.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the formula used to calculate the heat stress index.
**Option B:** This option is incorrect because it is not a recognized value for the heat stress index in severe heat strain.
**Option C:** This option is incorrect because it is not a valid calculation for the heat stress index.

**Clinical Pearl / High-Yield Fact**
The heat stress index is a valuable tool in assessing the risk of heat-related illnesses, especially in individuals working in hot and humid environments, such as construction workers or military personnel.

**Correct Answer:** C.