SAT Question 902dc959: Right Triangles And Trigonometry answer and explanation with Desmos solution

Choice C is correct. Angle $A$ is an acute angle in a right triangle, so the value of $\tan A$ is equivalent to the ratio of the length of the side opposite angle $A$, $20$, to the length of the nonhypotenuse side adjacent to angle $A$, $21$. Therefore, $\tan A=\frac{20}{21}$.