All squares are rectangles, so the answer would still be YES.hence C

Necessary condition for a quadrilateral to be a rectangle is its diagonals bisect each other and angles are of 90 degree.
Statement 1. We are given that the diagonals bisect each other but we are unaware of the angles. From statement 1 we can deduce that the quadrilateral could be a square or a rectangle or a rhombus. Hence, Inusfficient.
Statement 2. Any quadrilateral can have one of its angle equal to 90 degrees. Hence, Insufficient.
Statement 1 & 2 together. Combining the results of statement 1 & 2, we get,
A quadrilateral whose diagonals bisect each other and 1 angle is 90 degree. So, it is definitely a rectangle because when diagonals bisect each other and 1 angles is of 90 degree, then all other angles are also of 90 degree.
Hence, the given quadrilateral is a square or a rectangle.
We know, every square is a rectangle. Hence, the given quadrilateral is definitely a rectangle. Hence, sufficient.

Yes, but a rhombus is a parallelogram (A rhombus is a parallelogram with four congruent sides).
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Statement 1 tells us that the shape is a parallelogram, but it could be a square, rhombus, or rectangle

Statement 2 tells us that the shape has 1 90 degree angle.

Statement 1 + 2 tells us that it is a parallelogram that has 1 90 degree angle, so we can deduce the following:
- Other angles must also be 90 degrees as angles in parallelograms are supplementary
- the shape cannot be a rhombus as rhombi do not have 90 degree angles
This allows us to conclude that the shape is either a square or rectangle.
In GMATLand it is common to get tested on the trap that a square is actually a form of rectangle so keep this in mind.

Combined = sufficient.
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