How was convict labor used during the antbellumm era ?

Just like slavery.

Explanation:

After slavery was ended by the civil war, the southern States still treated the African American people with hostility. The criminal system that they created targeted mostly the African American community, which make their prison is heavily populated by ex-slaves.

They created a system called convict labor.

In this system, the prison "Rent" their prisoners to private companies to work as their labor. In return, the companies will pay the prison with a sum amount of money that is lower compared to hiring workers with normal process. Since they're convicts, the prison  in southern states did not reward the prisoners in any way. So, it works like slavery.

the answer your looking for is c. : )