the diameter of a sphere...
a is half the length of the radius
b is the distance from the center of the sphere to any point on the surface of the sphere
c is the longest distance on a sphere
d does not pass through the center of a sphere
The longest distance on a sphere is called the diameter. It is denoted by "c" in your question. The diameter of a sphere is twice the length of the radius.
So, we can write:
diameter (c) = 2 * radius