Jason is trying to prove that this quadrilateral is a rhombus. Using the slope formula, he finds that opposite sides of the polygon are parallel. Since all of the sides appear to be congruent, Jason concludes that ABCD is a rhombus. Is Jason's reasoning correct? Why or why not

Opposite sides being parallel proves that hte figure is a parallelogram.  The sides only appear to be equal so his conclusion is not valid. He would need to show that  2 adjacent sides of the parallelogram are equal ( using the distance formula)  to prove that it is a rhombus.