What is the formula to find the circumference of a circle?

The formula to find the circumference of a circle is based on the relationship between the circle's diameter and radius. The correct formula, Circumference = 2πr, indicates that the circumference is calculated by multiplying the radius (r) by 2 and then by π (pi), which is approximately 3.14. This formula relies on the definition of the radius as half the diameter, reinforcing the connection between the two measurements.

Additionally, the formula Circumference = πd is another valid representation, where d denotes the diameter of the circle, which is equal to 2r. This reflects that both formulas serve the same purpose; they are just expressed differently based on the radius or the diameter.

The other formulas provided do not represent the correct calculation of circumference. Specifically, the formula Circumference = πr² calculates the area of the circle, not its circumference, and the addition of 'd' in the last choice introduces an incorrect element, as circumference is not defined that way. Thus, the appropriate formulas for circumference are rooted in the relationship between diameter and radius as seen in the correct answer.