Most enslaved people who learned to read and write gained that knowledge

Most enslaved people who learned to read and write gained that knowledge on their own, in secret.

During the era of slavery in the United States,  literacy was believed to be a threat to the institution of slavery due to the fact that , literacy facilitated knowledge pertaining to the successful slave revolution in Haiti of 1791–1804, the end of slavery in the British Empire in 1833, and the writings of abolitionists. Furthermore, literacy allowed slaves to have better access to information regarding the Underground Railroad and other routes to freedom.

d is the correct answer

~courtney