What is the formula for the diameter of a circle?

The correct choice is derived from the fundamental relationship between the radius and the diameter of a circle. The diameter is defined as the longest distance across the circle, passing through the center, which is essentially twice the radius. Therefore, the formula for the diameter can be expressed as two times the radius.

This concept is built into the geometric properties of circles: if you take a circle and measure the distance from the center to any point on the circle (this distance is the radius), then the diameter is simply extending that distance all the way across the circle, which naturally leads to doubling the radius. Hence, the relationship is succinctly summarized as diameter equals two times the radius. This understanding is fundamental when solving problems involving circles in geometry.