The measure that does not change when a wave moves from one medium to another is the frequency. Frequency refers to the number of complete waves that pass through a given point in one second. It is a characteristic property of the wave and does not depend on the medium through which it travels.
On the other hand, speed and amplitude are affected by the medium. When a wave moves from one medium to another, its speed changes because the speed of a wave is dependent on the properties of the medium it is traveling through, such as its density and elasticity. The amplitude of a wave also changes when it moves from one medium to another because the amplitude is related to the amount of energy that the wave carries, which can be absorbed or reflected by the medium.
Therefore, it is only the frequency that remains constant when a wave moves from one medium to another. This property is important in various applications, such as radio and television broadcasting, where different frequencies are used to transmit different types of information.