Based on your response options, the most likely outcome that could be proved using a diagram is:
Alternate interior angles are congruent.
This is because a diagram illustrating two parallel lines cut by a transversal can visually demonstrate that the alternate interior angles formed are congruent. The other options, while they may relate to diagrams, typically require more than just a simple visual representation to prove, especially the Triangle Angle Sum Theorem or the concept of parallel lines having the same slope.