Enthalpy of combustion of methane, ethane, ethene and ethyne are -890, -1560, -1411 and -1314 kJ/mole respectively. Best fuel among them is

Best fuel has high calorific value (heat liberated per gram of substance)

$$\begin{gathered} \mathrm{Methane}=-\frac{890}{16}=-55.625 \mathrm{kJ}/\mathrm{g} ; \\ \mathrm{Ethane}=-\frac{1560}{30}=-52 \mathrm{kJ}/\mathrm{g} ; \\ \mathrm{Ethene}=-\frac{1411}{28}=-50.4 \mathrm{kJ}/\mathrm{g} ; \\ \mathrm{Ethyne}=-\frac{1314}{26}=-50.53 \mathrm{kJ}/\mathrm{g} ; \end{gathered}$$