The practice by which slave owners emancipated the slaves was known as the underground railroad manumission the 13th amendment ending of indenture

Manumission

Explanation:

Manumission is also called Affranchisement that is the act of a slave owner freeing his slaves. There were many ways to do that, sometimes slaves would save money for a long time until they could purchase themselves. Many states regulated manumission, what costs should be paid and how.

the correct answer is:

b) a rich merchant.