During sensible cooling of air, its wet bulb temperature

The correct answer is A: Decreases but dew point remains constant.
Let's break down why:
Sensible cooling means you're only changing the temperature of the air, not the amount of moisture in it.
Think of it like this: you're cooling the air, but you're not taking any water out.
Wet bulb temperature is the temperature a parcel of air would have if cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel.
When you cool the air *without* changing the amount of water vapor present, the wet-bulb temperature will also decrease.
The dew point is the temperature to which air must be cooled to become saturated with water vapor. Since the amount of moisture in the air is constant during sensible cooling, the dew point remains constant.
Therefore, wet bulb temperature decreases and dew point remains constant.