Final answer:
The wavelength of a wave increases if the wave is traveling at a constant speed and its frequency decreases, based on the wave equation relationship.
Step-by-step explanation:
If a wave is traveling at a constant speed, and the frequency decreases, the wavelength would increase. This is explained by the wave equation v = f × λ, where v is the wave speed, f is the frequency, and λ (lambda) is the wavelength. As wave speed is constant, a decrease in frequency must result in an increase in wavelength in order for the equation to remain balanced. Therefore, if the frequency of a wave decreases, the correct answer is A. It would increase.