Answer is C.
Given A = B + 5 and C = B – 4, and let B = x hours, so,
A = (x+5) hours and C = (x-4) hours
Condition, given in the question, A + B = C
$\frac{1}{x+5} + \frac{1}{x} = \frac{1}{x-4}$
$x^{2} -8x-20=0$
On solving the above quadratic equation, we will get value of x = -2, 10.
Acceptable value of x = 10
Hence the time required by the first pipe A = x + 5 = 15 hours