If a circle has a radius of 4, what is the diameter?

To determine the diameter of a circle when given the radius, you use the relationship between radius and diameter. The radius is the distance from the center of the circle to any point on its circumference, while the diameter is the distance across the circle, passing through the center.

The formula connecting radius to diameter is:

[ \text{Diameter} = 2 \times \text{Radius} ]

In this case, the radius is 4. Plugging this value into the formula gives:

[ \text{Diameter} = 2 \times 4 = 8 ]

This calculation shows that the diameter is 8, confirming the choice as the correct answer. Understanding this relationship is crucial in geometry, as it often comes up in various problems involving circles.