Mole fraction is defined as the amount (number of moles or molecules) of a constituent or component divided by the total amount of all the constituent or component present.
Mole fraction=TotalnumberofmolesofallthecomponentsNumberofmolesofcompnent
Suppose we have a mixture with 3 components A, B and C.
Mole fraction of C=nA+nB+nCnC
Hence, the answer is option B.