Which formula expresses the circumference of a circle?

The formula for the circumference of a circle is indeed represented by C = 2πr, where C stands for circumference and r represents the radius of the circle. This formula is derived from the relationship between the diameter of the circle and its radius. The diameter is twice the radius (d = 2r), and since the circumference is defined as the distance around the circle, it can be calculated by multiplying the diameter by π (C = πd). By substituting the diameter with its equivalent in terms of the radius, the formula is established.

In other contexts, the other formulas listed do not represent the circumference of a circle. For instance, C = πr² represents the area of a circle, which is a different concept altogether focused on the surface within the circle's boundaries rather than the distance around it. The equation C = r² + h² appears to resemble the Pythagorean theorem, which applies to right triangles and is not relevant to circles. Finally, C = 2r fails to incorporate π, indicating that it omits a fundamental aspect of circle geometry. Thus, the correct formula for the circumference is clearly C = 2πr.