• A. Amplitude
• B. Wavelength
• C. Velocity
• D. Frequency ✓

When a wave travels from one medium into another, the property that remains constant is the frequency. Frequency is defined as the number of complete oscillations or cycles of a wave that occur in a unit of time. When a wave moves from one medium to another, its frequency remains the same, while other properties such as wavelength and velocity may change.

Frequency is an intrinsic property of the wave itself and is determined by the source of the wave. As the wave moves from one medium to another, the frequency does not change because it is independent of the medium through which the wave is traveling. This means that regardless of the medium, whether it be air, water, or a solid material, the frequency of the wave will remain constant.

The reason for this phenomenon lies in the nature of waves and their behavior when encountering boundaries between different mediums. When a wave crosses into a new medium, its speed and wavelength may change due to differences in the properties of the two mediums. However, since frequency is directly related to the source that produces the wave, it remains unchanged during this transition.

In summary, when a wave travels from one medium into another, its frequency remains constant while other properties such as wavelength and velocity may vary.