What does the relative formula mass (Mr) represent?

The relative formula mass (Mr) represents the sum of the relative atomic masses of all the atoms present in a compound, which is why the answer you've chosen is correct. When calculating the relative formula mass, each element's atomic mass as given in the periodic table is multiplied by the number of times that element appears in the formula of the compound. This gives a measure of the mass of a mole of that compound in atomic mass units (u), allowing for comparisons between different substances based on their molecular weights.

This concept is fundamental in stoichiometry and helps chemists understand the proportions in which substances react, as well as in predicting the outcomes of chemical reactions and calculating yields. It ties back to foundational principles in chemistry, linking atomic and molecular levels of understanding to practical applications in laboratory work and industry.