In order for the wavelength of a wave to increase, the frequency must ...

a. stay the same

b. increase

c. decrease

d. increase or decrease, depending on the wave

Decrease. Think like this, if you were standing in the ocean getting hit by waves, frequency would be how many waves hit you in a certain amount of time. If there were lots of little waves hitting you the number of waves (the frequency) would be high, whereas big waves are slower and take more time to reach you so you would get hit by less of them in the same time frame. The less frequent, the longer the wave.