Question

As the frequency of a wave traveling at constant speed increases, what decreases?

Answer 1

There is an inverse relationship between frequency and wavelength. If the speed is held constant, as frequency increases, the wavelength must decrease.
Think of light waves traveling through space in a vacuum. The speed of light is $3.0\times {10}^{8}m/s$ in a vaccum.
If the light waves begin to strike an object more and more frequently, then the wavelength must be getting shorter.
You might imagine the color of the light shifting from red to blue since the wavelength of blue light is shorter than the wavelength of red light, even though they both travel at the same speed.