The circumcenter of a triangle is the point where the perpendicular bisectors of each side of the triangle intersect. Here, "perpendicular bisectors" are the lines that are perpendicular to the midpoint of each side of the triangle.
This circumcenter has a very special property: it is the center of a circle that can pass through all three vertices of the triangle. This circle is known as the circumcircle.
So, the correct answer to the question "The circumcenter of a triangle is also the center of" is:
A. a circle circumscribing the triangle
