Question

most enslaved people who learned to read and write gained that knowledge
from enslavers.
in plantation schools.
on their own, in secret.
from abolitionists in cities.

Explanation:

In the history of American slavery, most enslavers banned enslaved people from learning to read and write to maintain control. Formal plantation schools did not exist for enslaved people, and help from abolitionists or enslavers was rare. Enslaved people typically taught themselves or each other in secret to gain literacy skills.

Answer:

on their own, in secret.