Why does air at 75% relative humidity and 30°c make your skin feel stickier than air at 75% relative humidity at 10°c
a) warm air rises while cold air falls
b) the warmer air contains more water vapor per unit of volume
c) the warmer air contains more water vapor per unit of volume
d) moist air has lower densitdensity than dry air does

Option B, The warmer air contains more water vapor per unit of volume

Explanation:

The amount of water vapour contained in air depends on the temperature f air. A warm air has potential to hold more moisture as compared to cold air. Therefore when the temperature of air falls , its ability to hold moisture also falls.

An air with RH of 75% at 30 degree Celsius  can hold 75% of moisture at this particular temperature. As the temperature increases, the ability of air to hold moisture also increases.

Hence, option B is correct.