Final answer:
When a wave travels between media of different densities, it changes speed due to refraction, which is influenced by the properties of the new medium.
Step-by-step explanation:
When a wave travels between media of different densities, it experiences a change in properties. The correct answer to what happens when a wave travels between media of different densities is D. It changes speed. This phenomenon is due to the wave encountering different physical conditions in the new medium, such as different densities or elastic properties, which affects how fast the wave propagates.
For example, when a wave moves from a low-density medium to a high-density medium, it generally slows down. The frequency of the wave remains constant, but its wavelength and speed can change. This process is known as refraction. It is important to note that the type of the wave, whether it is transverse or longitudinal, does not inherently change when moving between different media, nor does a mechanical wave become an electromagnetic wave, nor does it change color due to this transition alone.