1. The problem asks: In which triangle can you apply Pythagoras' Theorem? 2. Pythagoras' Theorem applies only to right-angled triangles. It states: $$a^2 + b^2 = c^2$$ where $a$ and $b$ are the legs (perpendicular sides) and $c$ is the hypotenuse (the side opposite the right angle). 3. From the descriptions: - Diagram A has a right-angle square at the top vertex, indicating a right angle. - Diagram B has no right-angle marker. - Diagram C is a right triangle with right angle implied by tick marks on perpendicular sides. - Diagram D has tick marks on sides but no right-angle square. 4. Therefore, Pythagoras' Theorem applies to Diagram A and Diagram C only. Final answer: Diagrams A and C.