Part of the tank filled by Pipe A in 1 hour is $\frac{1}{5}$

Part of the tank filled by Pipe B in 1 hour is $\frac{1}{6}$

Part of the tank filled by pipe C in 1 hour is $\frac{1}{30}$

Net part filled in 1 hour when all pipes are opened together $\frac{1}{5}+\frac{1}{6}+\frac{1}{30}$

$=\frac{6+5+1}{30}=\frac{12}{30}=\frac{2}{5}$

Required time $=\frac{5}{2}=2\frac{1}{2} \mathrm{hours}$

Hence option B is correct