Answer: C. From the percentage by mass you can work out the mole ratio of the elements and hence the empirical formula — but to convert a percentage into moles you need the relative atomic masses (Aᵣ) of carbon, hydrogen and oxygen, not their atomic numbers.
The empirical formula only gives the simplest whole-number ratio. To find the molecular formula you also need the relative molecular mass (Mᵣ) of X, so you can work out how many empirical units make up one molecule.
So the information needed is the relative atomic masses of C, H and O and the relative molecular mass of X — option C. Options using atomic numbers (A, B) are wrong; option D omits Mᵣ and so cannot give the molecular formula.
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