Two right triangles are similar, with corresponding angle A in each triangle. In the smaller triangle, (A) = 3/5. What is (A) in the…

Similar triangles have equal corresponding angles, so angle $A$ is the same size in both triangles. A trigonometric ratio such as $\sin(A)$ depends only on the angle, not on how big the triangle is: scaling every side by the same factor multiplies the opposite side and the hypotenuse by that factor, and the factor cancels in the ratio. So $\sin(A) = 3/5$ in the larger triangle too, choice B. Choice A, $4/5$, is $\cos(A)$ for a 3-4-5 triangle, a mix-up between the opposite and adjacent sides. Choice C assumes the ratio changes with size, which it does not; the scale factor is not needed. Choice D, $9/25$, squares the ratio, as if a scale factor applied to area changed the sine.