Final answer:
The central idea of paragraph 1 is that Frederick Douglass's new mistress was initially kind, a trait Douglass found surprising and linked to her lack of prior experience as a slaveholder.
Step-by-step explanation:
The sentence that best describes the central idea of paragraph 1 is: c. Frederick Douglass's new mistress was kind when he first met her because she had not been a slaveholder before. Frederick Douglass expresses his astonishment at the kindness of his new mistress and attributes it to her inexperience with slavery. She treated Douglass as a human being rather than merely property, showing emotions and actions uncharacteristic of the dehumanizing institution of slavery at the time.