Which mathematical expression represents a circle?

The expression that represents a circle is A = πr². This formula is used to calculate the area of a circle, where "A" is the area, "π" (pi) is a constant approximately equal to 3.14, and "r" represents the radius of the circle. The radius is the distance from the center of the circle to any point on its circumference.

This formula specifically relates to the two-dimensional nature of a circle, illustrating how the space enclosed by the circle can be quantified. Understanding this formula is crucial in geometry, as it connects the linear dimension of the radius to the two-dimensional concept of area, allowing for practical applications in various fields such as physics, engineering, and everyday problem solving.

The other choices represent different aspects related to circles or dimensions but do not provide the correct formula for the area: A = 2πr is used to calculate the circumference, A = r² does not appropriately reflect the area of a circle, and A = πd relates to the circumference as well, using the diameter instead of the radius.