The correct answer is
B.) decrease
In fact, the relationship between frequency, wavelength and speed of a wave is given by
[tex]\lambda= \frac{v}{f} [/tex]
where
[tex]\lambda[/tex] is the wavelength
f is the frequency
v is the speed of the wave
The speed v of a wave depends only on the properties of the medium; in this problem the medium does not change, so the speed remains the same.
Instead, the frequency f increases: from the previous equation we see that the wavelength is inversely proportional to the frequency, therefore if the frequency increases, the wavelength will decrease.
