Place the figure in a coordinate plane in a convenient way.
Assign coordinates to each vertex.
Rectangle: length is l and width is b.

The correct answer is: D = (l, b)

Complete step by step solution:
                                        

Here we have ABCD as a rectangle placing one vertex as origin A .
We denote length of the rectangle by l and width by b.

Now, AB = l

CD = l (Since ABCD is a rectangle)

And, AC = b

BD = b (Since ABCD is a rectangle)

So we get that D = (l, b)