Answer:
On a side of the triangle.
Explanation:
The circumcenter of a triangle is the points formed by the intersection of the perpendicular bisector of the sides. This point can be located inside, outside, on a side, depending on the type of triangle.
In the special case of right triangles, the circumcenter is located on the midpoint of the hypothenuse, that is, on a side of the triangle, like the image attached shows.
The second image attached show the circumcenter.
Therefore, the correct answer is the first choice.