The solution would be like this for this specific problem:
ab/a+b = c
b = -1 + a
for c:
(5(-1 + 5))/(5 + (-1 +5))
5(4)/ 5 + 4
20 / 9
The correct answer between all the choices given is the first choice or letter A. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.