Trouton's ratio is given by (where _ b = molal heat of vaporisation of a substance at its normal boiling point, kcal/kmol Tb = normal boiling point, °K )

Answer: Option A

Troutons rule states that for most of the liquid the entropy of vaporisation is constant at boiling points and the entropy of vaporisation is given by ratio of enthalpy of vaporisation to normal boiling temperature $$\left(T_b\right)$$ nothing but troutons ratio.
$$\Delta S_{vap}=\frac{\Delta H_{vap}}{T_b}$$