**Explanation:**

To understand why the diameter of a circle is twice the radius, we need to understand what the terms "diameter" and "radius" mean in relation to a circle.

**Diameter:**
The diameter of a circle is a straight line that passes through the center of the circle and touches two points on the circumference. It is the longest chord of the circle.

**Radius:**
The radius of a circle is a straight line segment that connects the center of the circle to any point on the circumference. It is half the length of the diameter.

Now, let's compare the diameter and the radius:

**Statement: The diameter of a circle is twice the radius.**

To prove this statement, we can use the formula for finding the circumference of a circle:

**Circumference (C) = 2 * pi * r**

where "r" represents the radius of the circle.

If we substitute the value of the diameter (d) in the formula, we get:

**C = pi * d**

Since the circumference of a circle is the distance around the circle, it is equal to the length of the diameter.

So, from the formula, we can conclude that the diameter (d) is equal to the radius (r) multiplied by 2.

**Therefore, the correct answer is option 'D': Twice the radius.**

In summary, the diameter of a circle is always twice the length of the radius. This relationship is consistent for all circles, regardless of their size or position.