You have the following sentence:
triangle ABC is right triangle, so angle ∠A measures 90°
In order to find a counterexample of the previous sentence, you simply determine which of the given triangle is a right triangle in which the measure of angle ∠A is not 90°.
You can notice that the option B is a right traingle where the angle A does not measure 90°, otherwise, angle ∠B measures 90°
