"c" is the hypotenuse of the largest triangle, and "a" is that triangle's shortest side. Thus the ratio is
... (longest side)/(shortest side)
Since all the triangles in the figure are similar, to complete the proportion, you need to recognize the triangle that "a" is the longest side of, then find the shortest side of that triangle.
"a" is the longest side (hypotenuse) of the smallest triangle. The shortest side of that triangle is "r". So, your proportion is ...
... c/a = a/r
The appropriate choice is the 2nd one, ...
... r