Let the capacity of the storage tank be $x \;\text{litres}.$
$\begin{array}{lccc} & \textbf{P} & \textbf{Q} & \textbf{R} \\ \text{Time:} & 10\;\text{minutes} & 6\;\text{minutes} & \\ \text{Capacity of tank:} & x \;\text{litres} & & \\ \text{Efficiency:} & \frac{x}{10} \;\text{litres/minute} & \frac{x}{6} \;\text{litres/minute} & 34 \;\text{litres/minute} \end{array}$
If it takes one hour to completely empty a full storage tank with all the pipes operating simultaneously.
Now, $\frac{x}{10} \times 60 + \frac{x}{6} \times 60 = 34 \times 60$
$\Rightarrow \frac{x}{10} + \frac{x}{6} = 34$
$\Rightarrow \frac{6x + 10x}{60} = 34$
$\Rightarrow 16x = 34 \times 60$
$\Rightarrow {\color{Blue}{\boxed{x = 127.5\;\text{litres}}}}$
$\therefore$ The capacity of the storage tank (in litres) is $127.5.$
Correct Answer $:\text{D}$
${\color{Magenta}{\textbf{PS:}}}\;{\color{Green}{\boxed{\text{Total work = Time} \; \times\; \text{Efficiency}}}}$