Complete combustion of a hydrocarbon converts it to carbon dioxide and water, and the balanced equation fixes the stoichiometric ratio of oxygen consumed. For propane the equation is $C_3{H}_8+5O_2\to 3C{O}_2+4H_{2}O$. Balancing shows that one mole of propane requires exactly five moles of oxygen. At standard temperature and pressure one mole of any ideal gas occupies $22.4L$, so five moles of oxygen occupy $5\times 22.4=112L$. Hence $112L$ of oxygen are needed. The value $22.4L$ is wrong because it represents only one mole of oxygen, ignoring the stoichiometric factor of five. The value $56L$ corresponds to $2.5$ moles, which would come from an incorrectly balanced equation, so it is incorrect. The value $134.4L$ equals six moles and overestimates the oxygen requirement, so it does not fit. The reliability of the calculation rests on the law of conservation of mass, which guarantees that the coefficients in the balanced equation reflect the true mole ratios of reactants and products. Avogadro's law then ensures that equal moles of any gas occupy equal volumes under the same conditions, validating the direct conversion from moles to litres. This applies the NCERT combustion equation together with molar gas volume. A plausibility check: propane has eight hydrogens and three carbons, demanding substantial oxygen, and five moles giving $112L$ is consistent with the balanced reaction.