If a substance ' A ' dissolves in solution of a mixture of ' B ' and ' C ' with their respective number of…

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$=\frac{\mathrm{Number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{compnent}}{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{all} \mathrm{the} \mathrm{components}}$
Suppose we have a mixture with $3$ components A, B and C.

Mole fraction of $\mathrm{C}$$=\frac{{\mathrm{n}}_{\mathrm{C}}}{{\mathrm{n}}_{\mathrm{A}}+{\mathrm{n}}_{\mathrm{B}}+{\mathrm{n}}_{\mathrm{C}}}$

Hence, the answer is option B.