Answer:
![\text{C. }\cos R=21/29](https://img.qammunity.org/2022/formulas/mathematics/high-school/zzeqiqrt1lcqgvpq8wdklmrzqso80ptrik.png)
Explanation:
In any right triangle, the cosine of an angle is equal to its adjacent side divided by the hypotenuse.
From the diagram, we know:
- Angle R's adjacent side is 21
- The hypotenuse (longest side) of the right triangle is 29
Therefore, the cosine of angle R must be
![\boxed{(21)/(29)}](https://img.qammunity.org/2022/formulas/mathematics/high-school/axjd5zw5k0z3ccokczakep7a16ip7dyx55.png)