1. The problem asks: Identify the triangle where Pythagoras' Theorem can be applied. 2. Pythagoras' Theorem applies only to right-angled triangles. The formula is: $$a^2 + b^2 = c^2$$ where $a$ and $b$ are the legs and $c$ is the hypotenuse. 3. Since the user specifies "Only 1," we consider only one triangle. 4. The triangle must have a right angle to apply the theorem. 5. Therefore, the triangle with a right angle (Diagram A or C) is the correct choice. Final answer: Only one triangle with a right angle can use Pythagoras' Theorem.