Which of the following describes a right angle?

A right angle is defined as an angle that measures exactly 90 degrees. This specific measurement is critical in geometry and is often depicted as the square corner in various shapes such as rectangles and squares. The concept of a right angle is foundational in understanding other geometric principles, as many shapes and theorems relate directly to angles of this measurement.

The other options do not meet the criteria for a right angle. An angle that is less than 90 degrees is known as an acute angle, while an angle that exceeds 90 degrees but is less than 180 degrees is known as an obtuse angle. An angle measured exactly at 180 degrees is classified as a straight angle. Therefore, only the choice that specifies exactly 90 degrees correctly describes a right angle.