How can one calculate the diameter if the radius is known?

To determine the diameter of a circle when the radius is known, one must understand the relationship between these two measurements. The diameter is defined as the longest distance across the circle, passing through the center, while the radius is the distance from the center to any point on the edge of the circle.

When the radius is given, the diameter can be calculated by taking the radius and multiplying it by 2. This is because there are two radii in a full diameter; in other words, you need both halves of the circle's width to reach from one side of the circle to the other through its center.

This foundational concept of geometry is essential, especially in grade school mathematics, where understanding the properties of circles is aligned with the standards for this age group. Thus, multiplying the radius by 2 directly provides the correct measure for the diameter.

The other options presented do not correctly compute the diameter based on the radius, focusing instead on unrelated operations or incorrect calculations.