Which formula indicates how to calculate the circumference of a circle?

The formula for calculating the circumference of a circle is C = 2πr, where C represents the circumference, r is the radius, and π (pi) is a constant approximately equal to 3.14. This relationship shows that the circumference is directly related to the radius of the circle, meaning as the radius increases, so does the circumference.

While the other formulas presented relate to circles, they do not correctly define the circumference. The first option also represents the circumference of a circle, but in terms of diameter (C = πd), where d is the diameter, which equals 2r. The area of a circle is defined by the formula A = πr², and the last option (A = πd²) does not apply to circles at all. Therefore, the formula that captures the direct relationship between the radius of a circle and its circumference is C = 2πr, making it the correct choice for this question.