The correct answer is: C. Decrease.
The dew point is the temperature at which the air is saturated with water vapor and condensation begins. As the air temperature increases, the air can hold more water vapor. This means that the dew point will decrease as the air temperature increases.
Option A is incorrect because the dew point will not remain the same as the air temperature increases. The dew point will decrease as the air temperature increases.
Option B is incorrect because the dew point will not increase as the air temperature increases. The dew point will decrease as the air temperature increases.
Option D is incorrect because the dew point will not increase and become equal to the air temperature. The dew point will decrease as the air temperature increases.