When 10 ml of propane (gas) is combusted completely, volume of $C{O}_2\left(g\right)$ obtained in similar condition is

${{\mathrm{C}}_3}_{ }{{\mathrm{H}}_8}_{\left(\mathrm{g}\right)}+5{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to 3{\mathrm{CO}}_{2\left(\mathrm{g}\right)}+4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$

Each Propane molecule is made out of 3 Carbons and 8 Hydrogen atoms. During combustion, each Carbon is converted into a molecule of gaseous $C{O}_2$ and every pair of hydrogens into a water molecule. So, 3 Carbons of a Propane molecule gives 3 molecules of $C{O}_2$. Similarly, 10mL of Propane gives 30mL of $C{O}_2$.