Explanation:

- When it is registered: A will, even if registered, generally needs at least one attesting witness when being proved to establish its authenticity.

- When it is unregistered: An unregistered will also requires the testimony of an attesting witness to prove that it was properly executed and is valid.

- When it is admitted: If a will is admitted to probate or has been formalized, the presence of an attesting witness is still vital to prove the execution of the will.

- All of the above: Each of the above scenarios typically demands the presence of an attesting witness to vouch for the will’s legitimacy.

- Correct Answer: All of the above: It is necessary to call an attesting witness to prove a will in all the mentioned circumstances.