The correct answer is 12.5%.
Given, mass of solute (common salt) = 50 g
Mass of solvent (water) = 350 g
Mass of solution = mass of solute + mass of solvent
= 50 + 350 = 400 g
Mass percentage of solution = Mass of solute Mass of solution × 100 = 50 400 × 100 = 12.5% .