If a circle has a diameter of 10, what is its radius?

The radius of a circle is defined as half of its diameter. In this case, the diameter of the circle is given as 10. To find the radius, you simply divide the diameter by 2.

So, if the diameter is 10, the calculation would be:

Radius = Diameter / 2 = 10 / 2 = 5.

This shows that the radius of the circle is 5, making this option the correct choice. The relationship between diameter and radius is fundamental in circle geometry, as it serves as a basis for many calculations involving circles.