Which parts of this excerpt from Incidents in the Life of a Slave Girl by Harriet Ann Jacobs express the view that even "kind" slaveholders regarded their slaves as merely property?

I believe the answer is:

After a brief period of suspense, the will of my mistress was read, and we learned that she had bequeathed me to her sister's daughter, a child of five years old.

The words 'bequathed' means the act of passing ownership from one person to another. Even though the mistress is depicted as a kind owner, the fact that she can freely pass the ownership of a human being without her consent shows that the slaves simply seen as an object or property.

After a brief period of suspense, the will of my mistress was read, and we learned that she had bequeathed me to her sister's daughter, a child of five years old. So vanished our hopes ...