Final answer:
The frequency of a wave remains unchanged when it transitions between different mediums, while its speed and wavelength may vary.
Step-by-step explanation:
The wave property that stays the same as we change mediums is frequency. When a wave transitions from one medium to another, its speed and wavelength may change due to the different physical properties of the media, like density and elasticity. However, the frequency of the wave is determined by the source and remains the same when changing mediums. For example, if a sound wave moves from air into water, the speed and wavelength of the wave will adjust, but the frequency will remain constant as set by the source of the sound.