Answer:
b. relative humidity
Explanation :
The heat index , it is the measure of how the temperature feels to human body when the relative humidity is combined along the temperature of the air .
The body feels warmer in humid condition , due to the increased humidity the sweat will not evaporate and hence wont give cooling sensation to the body .
Similarly our body feels cooler in the arid conditions because the sweat evaporate quickly cooling our body.
Thus, as the air temperature and relative humidity increases the heat index also increase.