For a binary solution, the relation between the molality () of the solution and the mole fraction of — Solutions and Colligative Properties Chemistry Question

Question

For a binary solution, the relation between the molality ($m$) of the solution and the mole fraction of the solute ($\chi_B$) is given by which of the following expressions? (Where $M_A$ is the molar mass of the solvent $A$ in g/mol, and $\chi_A$ is the mole fraction of the solvent).