Southern apologists claimed the master - slave relationship was more humane than employer - worker relationships because

it afforded greater long-term security

Explanation:

it offered more opportunities for job training – this is not the correct answer. Slave apologists from the west didn’t care much for job training opportunities.

it afforded greater long-term security – this is the correct answer. Southerners who apologized slavery thought slaves actually benefited from this way of life where someone offers them control, plan for life, and structured job. In their ideas, slaves would be lost, frightened, even irresponsible if they had freedom. By enslaving them, they believed they gave them security and stability. This kind of life, they argued, is better than workers who had jobs and freedom in the North, as slaves had proper care and protection.

it was actually a "freer" relationship – this is not the correct answer. The southern apologists didn’t claim slaves were freer in this type of relationship, just that it was better for them.

it included women in the workforce – this is not the case. The problem of women's work rights was not mentioned at the time.