The purpose of the selma march was to campaign for

Voting rights for African Americans

Explanation:

The march was conducted in 1965, when protesters conducted a protest walk from Selma, Alabama to Montgomery. The purpose of the march is to protest racial injustice in united states and demanded the government to allow the african Americans to exercise their right to be involved in the democracy. (at that time they were already allowed to vote, but the government still created some barriers that prevent them to do it)

This march lead to the creation of the voting rights act 1965, which designed specifically to address those voting barriers.