The correct answer is: A. Decreases with increase of temperature.
The velocity of sound in air is inversely proportional to the square root of the absolute temperature. This means that as the temperature increases, the velocity of sound decreases. This is because the molecules in air are moving faster at higher temperatures, and they are therefore more spread out. This makes it more difficult for the sound waves to travel through the air, and so the velocity of sound decreases.
Option B is incorrect because the velocity of sound in air decreases with increase of temperature. Option C is incorrect because the velocity of sound in air does depend on temperature. Option D is incorrect because the velocity of sound in air does not increase with decrease of temperature.