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

The radius of a circle is defined as half of the diameter. In this case, since the diameter of the circle is given as 10, you can find the radius by dividing the diameter by 2.

So, 10 (the diameter) divided by 2 equals 5. This calculation leads us to understand that the radius of the circle is 5. This fundamental relationship between the diameter and the radius is consistent across all circles: the radius will always be half the length of the diameter. Therefore, the correct answer is 5.