Which of the following describes a wave's behavior when it slows down in a denser medium?

When a wave travels from one medium to another and slows down due to an increase in density, it undergoes a phenomenon known as refraction. This behavior occurs because the speed of the wave is affected by the properties of the medium it is entering.

Refraction leads to a change in the direction of the wave as it moves into the denser medium. The degree of bending depends on the difference in density and the speed of the wave in each medium. For example, light waves moving from air into water slow down and bend towards the normal line, which is an imaginary line perpendicular to the surface between the two media.

In contrast, reflection involves a wave bouncing back into the original medium after hitting a barrier, diffraction refers to the bending and spreading of waves around obstacles or through openings, and transmission pertains to the passage of the wave through the new medium without any bending. In this case, since the question specifically states the behavior of a wave slowing down in a denser medium, refraction is the most accurate description of the phenomenon.