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The correct formula for calculating the number of moles from mass and molar mass is found by rearranging the fundamental relationship between mass, moles, and molar mass. The concept is rooted in the definition of moles, where one mole of a substance is the amount that contains Avogadro's number of entities (atoms, molecules, etc.) and it corresponds to a specific mass related to its molar mass.

To find the number of moles, you divide the mass of the substance by its molar mass. Molar mass is expressed in grams per mole (g/mol), so when you take the total mass (in grams) and divide it by the molar mass, you get the number of moles. This formula effectively captures the ratio between the mass of the substance you have and the mass of one mole of that substance.

This concept reflects the relationship that as you have more mass, you increase the number of moles, provided the molar mass remains constant. Therefore, using mass divided by molar mass gives you the correct calculation for the number of moles. The reasoning behind the other provided options does not align with this established relationship in chemistry.