The constitution addressed slavery by: referring numerous times to "slaves" or "slavery" counting slaves as three fifths of a person for the purposes of apportionment requiring that all slaves count toward a state's congressional representation making it legal in every state requiring that slaves have full legal protections

The correct answer is counting slaves as three fifths of a person for the purposes of apportionment.

The fight over whether or not slaves would count towards a states population had major implications. This is because in one of the houses of Congress (the House of Representatives) the number of representatives in the state depends on its population. States with larger populations would then have more representatives. More representatives means that states would have more influence over national laws.\

This is why southern states (that had slaves) fought to count them towards the population while states without slavery fought against counting slaves as part of the population.