The relationship between the speed $v$, frequency $f$, and wavelength $\lambda$ of a wave is given by the equation:

$v = f \lambda$

If the speed $v$ of the wave is constant and the wavelength $\lambda$ increases, the frequency $f$ must decrease to keep the product $f \lambda$ constant. This is because the two variables are inversely related: when one increases, the other must decrease for the speed to remain the same.

Thus, the correct answer is:

D. The frequency decreases.

Would you like more details or have any questions?

Here are 5 related questions to expand on this concept:

1. How does the speed of sound change with temperature, assuming constant frequency?
2. What is the relationship between energy and frequency in a wave?
3. How do amplitude and frequency relate to each other in terms of wave properties?
4. What happens to a wave's frequency if both speed and wavelength are doubled?
5. Can a wave's frequency change without affecting its speed?

Tip: Remember, frequency and wavelength have an inverse relationship if the wave speed stays constant.