What is the empirical formula for a compound that is 31.9% potassium, 28.9% chlorine, and 39.2% oxygen?

The  empirical formula  for a compound   is KClO3

     Explanation 
 find  the  moles of each  element
moles  = % composition/molar  mass

molar  mass of  of  potassium =39g/mol ,chlorine = 35.5 g/mol, oxygen =16 g/mol

moles  of potassium  =  31.9 / 39 = 0.818  moles
moles of  chlorine    = 28.9/35.5 = 0.814 moles
moles of   oxygen  =  39.2/  16 =  2.45  moles

find the  mole ratio  by  dividing   with  the smallest  mole = 0.814  moles

potassium = 0.818/0.814  =1  
chlorine  =  0.814/0.814 = 1
oxygen =  2.45 /0.814 =3 

the empirical  formula   is therefore = KClO3

The answer is b metal denting