A circle or oval, dragged across the box that encloses it.
The Ellipse draws a closed curve inside a box you drag. It is the usual way to ring a structure or a detail without hiding what is underneath.
Draw it: press at one corner of the enclosing box, drag to the opposite corner, release. The ellipse touches the middle of each side of that box, so the drag sets the widest and tallest points rather than the outline itself.
Rectangle where the thing being ringed is rectangular; Cloud Ellipse for a revision set.