ANSWER:
b- halve the wavelength of the wave
Explanation:
We have that the speed of a wave is dependent upon the properties of the medium through which the wave is moving. An alteration in the properties of the medium will result in a change in the speed at which the wave moves through that medium.
That is to say that when changing frequency, what changes is the wavelength.
They are inverse, so when the frequency doubles, the wavelength is divided in half.
Therefore, the correct answer is:
b- halve the wavelength of the wave