Radius is defined as which of the following?

• A. Straight line extended from the center of a circle to the curved surface
• B. Straight line which touches the outside of a circle at only one point
• C. The point outside of a circle where a tangent line touches
• D. Straight line joining two points on a curve

Answer: (A)

Radius is the line segment from the center of a circle to any point on its circumference. That’s exactly what the option describes: a straight line from the center to the curved surface. This reflects the defining distance from the center to the boundary, and all radii in a circle have equal length.

The other ideas refer to different concepts: a tangent line touches the circle at one point but doesn’t necessarily pass through the center; a point outside touching the circle involves tangents from an external point; and a line joining two points on a curve is a chord, not a radius.