What allows the prediction of how molecules will combine in reactions?

The correct choice emphasizes the significance of the law of conservation of mass in predicting how molecules will combine in reactions. This fundamental principle states that in a closed system, the total mass of reactants must equal the total mass of products. This means that during a chemical reaction, atoms are neither created nor destroyed; they are simply rearranged to form new substances.

By understanding the conservation of mass, chemists can predict the quantities of reactants needed and the expected products formed when molecules combine. This principle also aids in balancing chemical equations, ensuring that the number of each type of atom is the same on both the reactants and products sides, which is essential for accurate predictions in reactions.

This foundational concept underlies much of stoichiometry and is crucial for any calculations involving chemical reactions, allowing scientists to make informed predictions about how substances will interact based on the mass of the reactants present.