When the frequency of a wave increases, the wavelength decreases. This is based on the wave relationship defined by the equation \( v = f \lambda \), where \( v \) is the wave speed, \( f \) is the frequency, and \( \lambda \) is the wavelength. If the speed of the wave remains constant and the frequency increases, the wavelength must decrease to maintain the equality.
Therefore, the correct response is:
The wavelength decreases.