If the radius of circle is r, then the diameter = ____.

The diameter is the longest chord that passes through the center of the circle. 

A radius connects the center of the circle to any point on the circle.
In the below example, AB is the diameter of the circle, and OA and OB are the radii of the circle. 

Hence we can say that the diameter is twice the length of a radius.
Therefore, if the radius is r then the diameter will be 2r.