In quadrilateral PQRS the diagonals PR and QS bisect each other and are equal in length. Which single extra fact would force PQRS to be a square?

Diagonals that bisect each other make PQRS a parallelogram, and equal diagonals then upgrade it to a rectangle. To become a square a rectangle needs equal adjacent sides, and that happens exactly when its diagonals also cross at right angles. The other three options add nothing new: a rectangle already has four right angles, its opposite sides are already parallel, and fixing the diagonal length at 10 cm merely sets the size while leaving the shape free to be any rectangle.