Question

All four sides are equal in length.
All four angles are 90°.
The diagonals are equal in length and intersect at right angles.
Opposite sides are parallel.

Answer 1

Those properties uniquely describe a square.

Reason: "All four angles 90°" makes it a rectangle; "all four sides equal" makes it a rhombus. A quadrilateral that is both a rectangle and a rhombus is a square. The equal, perpendicular diagonals and parallel opposite sides are additional characteristic properties of a square.