How many moles of $H_2$ will be needed to produce 6 moles of $H_{2}O$ when it reacts with excess of $O_2$ ?

The correct option is C 6
The balanced chemical equation can be written as,
$2H_2$ + $O_2$ $\to$ $2H_{2}O$
2 mol 1 mol 2 mol
So, to form 6 moles of $H_{2}O$, 6 moles of $H_2$ will be required.