Answer:
It is correct. Alan can tell by looking at the graph that all sides are congruent.
Explanation:
The length of a side can be found from the Pythagorean theorem. The square of a side length of the figure is the sum of the squares of the sides of the right triangle of which the figure's side is a hypotenuse. In each case, those right triangles have side lengths of 1 and 4, so the figure has side lengths that are ...
√(1² + 4²) = √17
All the figure's sides have this geometry, so they are all √17 in length. The fact that all sides are congruent is what makes the figure a rhombus. (The fact that sides are parallel is additional confirmation.)