According to the question,
All the angles of a quadrilateral are equal.
Suppose all the angles of the quadrilateral = x
We know that,
Sum of all angles of a quadrilateral = 360°
⇒ x + x + x + x = 360°
⇒ 4x = 360°
⇒ x = 360°/4
⇒ x = 90°
Hence, the quadrilateral is a rectangle.