How many molecules of oxygen are required for complete combustion of 1 mole of acetylene $\left(C_2{H}_2\right)$?

The correct option is C

Start with the balanced equation.

$2C_2{H}_2+5O_2\to 4C{O}_2+2H_{2}O$

$\therefore$ 1 mole of $C_2{H}_2$ requires $\frac{5}{2}$ moles of $O_2$ for complete combustion

$1mole$ of $O_2$
= $6.022\times {10}^{23}moleculesof{O}_2$

$\frac{5}{2}mole$ of $O_2$

= $\frac{5}{2}\times 6.022\times {10}^{23}$ molecules of $O_2$

= $15.055\times {10}^{23}$ molecules of $O_2$