Answer: 25%m/m
Explanation: The concentration of a solution (in mass percent) is the mass of the solute divided by the mass of the solution, in the given case, the solute is NaCl and its mass is 50.0 g, and the mass of the solution will be the mass of the solute plus the mass of the solvent (water):
mass of the solution= mass of NaCl+ mass of water
mass of the solution=50.0 g+ 150.0 g
mass of the solution=200.0 g
concentration (%m/m)=100*mass of the solute/mass of the solution
concentration (%m/m)=100*50.0g/200.0 g
concentration (%m/m) =25%m/m