Explanation:

The correct answer is option 'C' - diameter.

Definition of Diameter:
The diameter of a circle is a straight line segment that passes through the center of the circle and connects two points on the circumference. It is the longest chord in a circle.

Relationship between Diameter and Radius:
The radius of a circle is a straight line segment that connects the center of the circle to any point on the circumference. The diameter of a circle is simply twice the length of the radius. In other words, the diameter is the length of a straight line passing through the center of the circle and having both endpoints on the circumference.

Proof:
Let's assume the radius of the circle is 'r'.
Therefore, the diameter of the circle can be represented as '2r' since it is twice the length of the radius.

Example:
For example, if the radius of a circle is 5 units, then the diameter would be 2 * 5 = 10 units.

Importance of Diameter:
The diameter is an important measurement in a circle as it helps in calculating the circumference, area, and other properties of the circle. The diameter is also used to determine the size and scale of the circle in various applications such as construction, engineering, and mathematics.

Conclusion:
In summary, the diameter of a circle is two times the radius. It is a straight line segment passing through the center of the circle and connecting two points on the circumference.