Given that, $A\quad B \quad C$
Time $:\quad 12\quad 15\quad 6\quad$ minues
Total tank capacity $:LCM(12,15,6) = 60$ litre
Efficiency $: 5 \quad 4 \quad -10\quad $ litre/minute
Tap $A$ and $B$ open for $5$ minutes, then they can fill $5 \times 9 = 45$ litre
Now, all three taps are open, then the time taken to empty the tank is $\dfrac{45}{1} = 45$ minutes.
So, the correct answer is $(B).$