Option c
Let A is 1st pipe B 2nd and C 3rd pipe
Let B can do work in x hr
A can do work in x+5 hr
C can do work in x-4 hr
A can fill tank in 1 hr is 1/x+5 work
B can fill tank in 1 hr is 1/x work
A and B together can fill tank in 1 hr is (1/x) + 1/(x+5)
=(2x+5)/(x2+5x) work
A and B can together fill the tank in (x2+5x)/(2x+5) hr
(which is equal to the time C cn fill the tank alone)
X-4= (x2+5x)/(2x+5)
By solving it x=10 hr
So A cn fill the tank in 10+5= 15 hr