The correct answer is option 'A', Radius.

The line drawn from the center of a circle to any point on its circumference is called the radius. The radius is an important concept in geometry and is one of the fundamental properties of a circle.

Definition of Radius:
The radius of a circle is defined as the distance from the center of the circle to any point on its circumference. It can also be thought of as the line segment with one endpoint at the center and the other endpoint on the circumference.

Properties of the Radius:
1. Length: The length of the radius is the same for all points on the circumference of the circle. This means that any two radii of the same circle are congruent (have the same length).

2. Half of the Diameter: The radius is half the length of the diameter of a circle. The diameter is a line segment that passes through the center of the circle and has both endpoints on the circumference. Therefore, the radius is the distance from the center to the midpoint of the diameter.

3. Measure of Angles: The radius is also used to measure angles in a circle. An angle with its vertex at the center of the circle and its sides passing through two points on the circumference is called a central angle. The measure of a central angle is equal to the measure of the intercepted arc (the part of the circumference enclosed by the angle).

Example:
Consider a circle with center O and radius r. Let A be a point on the circumference. The line segment OA is the radius of the circle.

In this example, the line segment OA represents the radius of the circle. It is the distance from the center O to the point A on the circumference.

Therefore, the correct answer is option 'A', Radius.