Mole fraction is defined as the ratio of moles of a component to the total moles of all components in the mixture. Total moles (= 4 + 1 = 5) mol. Mole fraction of nitrogen (= 4/5 = 0.80). The mole fraction is dimensionless and ranges from 0 to 1. Option 0.20 is the mole fraction of argon ((1/5)), not nitrogen — the values are transposed. Option 0.25 would arise from an incorrect ratio of (1/4) (moles of argon to moles of nitrogen), which is not the definition of mole fraction. Option 0.75 has no arithmetic justification with the given data. This question directly tests the NCERT definition of mole fraction as a concentration expression for mixtures. Note that the sum of mole fractions in any mixture must equal unity: (0.80 + 0.20 = 1.00), which confirms the answer. Plausibility check: since nitrogen is the dominant component (80% by moles), a mole fraction of 0.80 is physically reasonable and greater than 0.5.