What would a hard bounce refer to on an email sending report?

What would a hard bounce refer to on an email sending report?
A . An email that is recognized, but returned to the sender because the recipient’s mailbox is full.
B . An email that is recognized, but returned to the sender because the mail server is temporarily unavailable
C . An email that was sent to a prospect marked as opted out because they visited the unsubscribe page.
D . An email that permanently bounced back to the sender because the address is invalid.

Answer: D