Concept: Trigonometric ratios in a right triangle.

Explanation of the Correct Answer: The correct answer is C. In option C, angle $R = 30°$ and the right angle is at $Q$, so angle $P = 180° - 90° - 30° = 60°$. We know $\cos 60° = \dfrac{1}{2}$.

Why the Other Options are Incorrect: A: Without angle information, $\cos P = \dfrac{QR}{RP}$ cannot be determined to equal $\dfrac{1}{2}$ from the given side lengths alone ($\dfrac{38}{76} = \dfrac{1}{2}$ but $QR$ is the side adjacent to angle $R$, not angle $P$). B: Angle $R = 60°$ gives angle $P = 30°$, so $\cos P = \cos 30° = \dfrac{\sqrt{3}}{2} \neq \dfrac{1}{2}$. D: Side lengths do not yield $\cos P = \dfrac{1}{2}$.

Vietnamese Version: Khái niệm: Tỉ số lượng giác trong tam giác vuông. Giải thích đáp án đúng: Đáp án C. Góc $P = 60°$, $\cos 60° = \dfrac{1}{2}$. Sai: B cho $\cos 30° = \dfrac{\sqrt{3}}{2}$.

Key Formula: $\cos(\theta) = \dfrac{\text{adjacent}}{\text{hypotenuse}}$