Which formula best represents the circumference of a circle in terms of diameter?

• A. A = πr^2
• B. C = 2πr
• C. V = πd^2
• D. C = πd

Answer: (D)

Circumference can be expressed directly in terms of diameter because the standard formula in radius form is C = 2πr, and the diameter d equals 2r. Replacing r with d/2 gives C = 2π(d/2) = πd, so the circumference is π times the diameter. This aligns with the idea that π links a circle’s size measured across (the diameter) to its boundary length (the circumference). The other expressions mix up what is being measured or use the wrong variable: A = πr^2 gives area using the radius, B = 2πr is the radius-based circumference, and C = πd^2 isn’t a correct formula for circumference, area, or volume.