Which of the following writers remarked that “You are your best thing”?

The correct option is Toni Morrison in Beloved.

Detailed Explanation:
The quote “You are your best thing” is one of the most celebrated and emotionally resonant lines from Toni Morrison's Pulitzer Prize-winning 1987 novel, Beloved.
In the narrative, this line is spoken by the character Paul D to the protagonist, Sethe, near the end of the book. Sethe, a survivor of slavery, has spent her life defined by her trauma and her maternal instinct to protect her children, often at the cost of her own identity and sense of self. When Sethe laments the loss of Beloved, whom she considered her "best thing," Paul D gently corrects her, helping her realize her own intrinsic self-worth and humanity independent of her past struggles and roles.