D. The centroid is also the center of mass, or where to balance a triangle of uniform thickness and density.
The circle inscribed in the triangle is called the incircle and its center the incenter. It's tangent to all three sides. There are three other triangles tangent to all three sides viewed as lines, called the excircles.
The circle circumscribing the triangle is the circle through the three vertices. It's called the circumcircle and its center the circumcenter.
The centers of each of these circles has a name, and all are known as triangle centers. The other common triangle center is the orthocenter, the meet of the altitudes.
The centroid and the orthocenter don't have associated circles.
