What is the formula for calculating the circumference of a circle?

The formula for calculating the circumference of a circle is correctly identified as 2πr. This formula highlights that the circumference is directly proportional to the radius of the circle. By multiplying the radius by 2π, you account for the complete distance around the circle since π represents the ratio of the circumference to the diameter.

Additionally, it's important to note that this relationship stems from the properties of circles where the circumference can also be derived using the diameter (which is twice the radius) multiplied by π, leading to the equivalent formula of πd. Within this context, understanding that the radius is a fundamental aspect of a circle allows for the intuitive grasp of these formulas.

Other options do not accurately represent the concept of circumference. For example, while one option suggests using the radius squared (r²), this actually represents the area of a circle rather than the distance around it. Another option includes terms that mix radius and diameter in a way that doesn't conform to the established mathematical definition of circumference. Thus, focusing on the correct representation reinforces the foundational principles of circle geometry.