Final answer:
Lassiter's interaction with Fay, the kind and human treatment by a mistress toward her slave, and the genteel manners of Elizabeth are instances of polite and civilized treatment.
Step-by-step explanation:
The character whose treatment of Liza is the most polite and civilized appears to differ across the pieces, as we are looking at different contexts and characters. However, there are a few instances where individuals exhibit notably polite and civilized behavior. For instance, Lassiter's gentle and loving assurance to Fay demonstrates a devoted and respectful demeanor, indicative of courteous treatment towards another character. There is a passage illustrating the kind treatment of a slaveholder to her slave, discussing her initial kindness and humanity before being corrupted by the institution of slavery, which shows a regard for treating people as human beings. Lastly, the genteel manners displayed by Elizabeth as well as her father's reputation for portrait painting suggest a background and behavior that align with being polite and civil in society.