What is the empirical formula of a compound containing 40.0% sulfur and 60.0% oxygen by mass?
s2o
so
so2
so3

in order to determine empirical formula we have to determine the mole ratio of the given elements

Let the total mass of the compound is 100g

as given that the compound has 40% sulfur , so mass of sulfur = 40g

as given that the compound has 60% oxygen, so mass of oxygen = 60g

let us calculate the moles of each element

Moles of sulfur = mass / atomic mass = 40 / 32 = 1.25

moles of oxygen = mass / atomic mass = 60/ 16 = 3.75

In order to get simple ratio of moles we will divide both the moles with least number of moles which is 1.25

moles of sulfur = 1.25 / 1.25 = 1

moles of oxygen = 3.75 /1.25 = 3

So empirical formula will be SO₃

i think it's letter a.