1. **Problem statement:** Find the measure of angle $a$ in a right triangle where the hypotenuse is 9.8 cm and the adjacent side to angle $a$ (bottom side) is 5.3 cm. 2. **Formula used:** In a right triangle, the cosine of an angle is the ratio of the adjacent side to the hypotenuse: $$\cos(a) = \frac{\text{adjacent}}{\text{hypotenuse}}$$ 3. **Apply the formula:** $$\cos(a) = \frac{5.3}{9.8}$$ 4. **Calculate the ratio:** $$\cos(a) = 0.5408$$ 5. **Find angle $a$ by taking the inverse cosine:** $$a = \cos^{-1}(0.5408)$$ 6. **Calculate the angle:** $$a \approx 57.3^\circ$$ 7. **Conclusion:** The measure of angle $a$ is approximately $57.3^\circ$. **Answer:** D 57.3°