91Ó°ÊÓ

In a rectangle, pair of opposite sides are congruent that implies pair of opposite sides has the same measure and all angles have same measure of $90°$.

In a square, all the sides are congruent that implies all sides have same measure and all angles have same measure of $90°$.

It is being given that MNOP is a rectangle.

In a rectangle, pair of opposite sides are congruent that implies pair of opposite sides have same measure and all angles have same measure of $90°$.

Therefore, $MN=OP$and $MP=NO$and all angles have same measure of $90°$.

It is also being given that:

$MN=NO$.

Therefore, it can be obtained that:

$MN=NO=OP=PM$

Therefore, all sides of the quadrilateral are congruent and all angles have same measure of $90°$.

Therefore, the quadrilateral MNOP is a square.

The most descriptive name for quadrilateral MNOP where MNOP is a rectangle with$MN=NO$ is a square.