Based on the options provided, it seems that the most likely statement that could be proved using a diagram involving geometric concepts is "Alternate interior angles are congruent." This statement is a fundamental property of parallel lines and transversals, which can often be visually demonstrated in a diagram.
While the other options such as the Pythagorean Theorem or Triangle Angle Sum Theorem can also be illustrated graphically, they typically rely on different types of diagrams or configurations. Therefore, "Alternate interior angles are congruent" is the best answer in this context.