Which of the following is a synonym for normal incidence?

Normal incidence refers to the situation where an ultrasound beam is directed perpendicular to the interface between two media. This means that the angle of incidence is exactly 90 degrees relative to the surface. In this context, the term "right angle" specifically denotes this 90-degree orientation, making it the synonym for normal incidence.

The other terms represent different angles of incidence. "Parallel" would imply an angle of 0 degrees, meaning the beam is travelling in the same direction as the boundary surface, rather than perpendicular to it. "Oblique" refers to angles that are neither perpendicular nor parallel, typically greater than 0 degrees and less than 90 degrees. "Acute angle" indicates any angle less than 90 degrees, which also does not describe normal incidence. Thus, "right angle" is the most appropriate term to describe the perpendicular interaction that characterizes normal incidence.