The correct statement is:
The frequency remains constant.
When a wave transitions from one medium to another (for example, from air to water), the frequency of the wave does not change. However, the speed and wavelength of the wave do change based on the properties of the new medium. The relationship between speed, frequency, and wavelength is given by the equation:
\[ \text{Speed} = \text{Frequency} \times \text{Wavelength} \]
Thus, if the speed changes because the wave is in a different medium, the wavelength must also change to keep the frequency constant.