Answer:
Lowering the humidity of the air.
Explanation:
The mathematical formula for the speed of sound wave is given by :

Where
is the adiabatic constant
T is the temperature of air
M is the molecular mass of the gas
or

B is the bulk modulus
is the density of the air
As the humidity of the air increases, then the number of water molecules increase. As a result, molar mass decreases and ultimately the speed of sound increases and vice versa.
So, the correct option is (A) " lowering the humidity of the air ". Hence, this is the required solution.