Solving for the percent composition by mass of sulfur. The formula would be: %S = mass of sulfur / mass of compound x 100% Plugging in our data: %S = 32 amu / 120 amu x 100% %S = 0.266666 x 100% %S = 26.66% Rounded off to two significant numbers it will be 27%. Therefore the answer is number 2.