Mole fraction is defined as the ratio of the number of moles of a component to the total moles of all components in the mixture. Moles of water = 36/18 = 2.0 mol. Moles of ethanol = 92/46 = 2.0 mol. Total moles = 2.0 + 2.0 = 4.0 mol. Mole fraction of ethanol = 2.0 / 4.0 = 0.500. Option 0.667 would result if the student calculated 2/(2+1) = 0.667 using incorrect moles for one component. Option 0.333 is the mole fraction of water in a 1:2 water:ethanol mole ratio, not what is asked. Option 0.720 comes from incorrectly taking mass fractions instead of mole fractions — mass of ethanol / total mass = 92/128 = 0.719 ≈ 0.720. It is important to distinguish mole fraction from mass fraction; the former uses moles while the latter uses masses. This calculation appears directly in NCERT Chapter 2 and forms the basis for applying Raoult's law and other mixture calculations. Plausibility check: since both components have equal moles here, the mole fractions are equal (0.5 each), which correctly sums to 1.0.