Flat Angle Definition: Understanding the 180-Degree Angle in Geometry

To solve this problem, let's begin by understanding what a flat angle is:

• A flat angle is an angle that measures exactly $180^\circ$.
• Visually, you can think of a flat angle as a straight line, where the two arms of the angle lie on opposite sides of the vertex, forming a straight angle.
• It is larger than a right angle, which measures $90^\circ$, and exactly twice the measure of a right angle.
• Flat angles are often used to understand the concept of supplementary angles, where two angles sum up to create a flat angle of $180^\circ$.
• None of the sides of a flat angle are considered to turn; they continue in a straight direction.

Among the given choices, the option that correctly defines a flat angle is the one that states, "Angle of 180°."

Therefore, the solution to the problem is that a flat angle is an angle of $180^\circ$.