The correct answers are: The suspect locked himself inside the bank vault by accident. The mayor herself provided an ironclad alibi for the prime suspect. The criminal mastermind planned the entire operation by himself.

...locked himself... correctly uses a reflexive pronoun because the subject ("The suspect") and the object are the same.

The mayor herself... correctly uses an intensive pronoun to emphasize that it was the mayor, surprisingly, who provided the alibi.

...by himself. correctly uses the phrase meaning "alone."

Incorrect options:

"The fleeing thief took the stolen diamonds with himself" is incorrect. For prepositions indicating spatial relationships (like with, beside, behind), we use personal object pronouns, not reflexive ones. It should be with him.

"...hid ourself..." is incorrect. "Witnesses" is plural third-person, so the correct reflexive pronoun is themselves. Furthermore, "ourself" is not a standard word; the first-person plural is "ourselves."