The two diagonals of an inscribed quadrilateral meet at the center of a circle.

The quadrilateral must be a

Sqaure

rhombus

rectagle

trapezoid

It must be a rectangle, as it is not possible for a rhombus to be inscribed, and a trapezoid could have one of its sides passing through the center, making it impossible to have its diagonals intersection be the center of a circle.