What is the diameter?
a.
5 mm
b.
10 mm
c.
20 mm
d.
100 mm

I believe the diameter will be D because when you find diameter you have to times the radius by the radius, you are trying to find the area of the circle.

The radius is the distance from the center of a circle to the perimeter. That is what is shown here.

The diameter is the distance across the circle when the line passes through the center. So the diameter will always equal twice the radius.

In your figure...

radius = 10 mm so
diameter = 2×r = 2 × 10 =

20 mm