Given that Darnell wants to be sure that the piece of wood he is cutting is in the shape of a rectangle, you need to remember the following properties of a rectangle:
1. Its opposite sides are parallel.
2. Its opposite sides are congruent (they have the same length).
3. Each interior angle measures 90 degrees.
4. Its diagonals are congruent.
5. Its diagonals bisect each other (they divide each other into two equal parts).
Then, when a parallelogram has all those properties, you can conclude that it is a rectangle.
In this case, Darnell can be sure if the piece is in the shape of a rectangle if the diagonals of the piece of wood are congruent.
Hence, the answer is: Last option.