Final answer:
To solve for R in the equation D = 2r, divide both sides of the equation by 2 to get R = D/2.
Step-by-step explanation:
To solve for R in the equation D = 2r, we need to isolate R on one side of the equation. We can do this by dividing both sides of the equation by 2. This gives us:
R = D/2
Therefore, the correct answer is R = D/2 (Option A).