Both are technically correct, but the choice depends on the style guide you're following:
1. Donna Davis's
* Preferred in most modern styles, like APA, Chicago, and MLA.
* Example: "Donna Davis's family keeps hawks."
2. Donna Davis'
* Used in older or more traditional styles, or when trying to keep it simple, especially with names ending in "s."
* Example: "Donna Davis' family keeps hawks."
For clarity and modern usage, Donna Davis's is generally better.