To develop this problem it is necessary to apply the concept related to the speed of sound waves in fluids.
By definition we know that the speed would be given by
Bulk modulus
Density of air
From the expression shown above we can realize that the speed of sound is <em>inversely proportional</em> to the fluid in which it is found, in this case the air. When the density increases, the speed of sound decreases and vice versa.
According to the statement then, if the density of the air decreases due to an increase in temperature, we can conclude that the speed of sound increases when the temperature increases. <u>They are directly proportional.</u>