The correct answer is option 2, Habeas corpus. Habeas corpus is a legal term that refers to a writ (a written order) issued by a court to a person or entity (such as a prison warden or government official) holding someone in custody. The purpose of the writ is to bring the detained person before the court in order to determine the legality of their detention. If the court finds that the detention is illegal or without proper authority, they can order the release of the person.

This option is correct because a writ of habeas corpus is specifically used to secure the release of a person who has been detained illegally. It is an important legal tool to ensure that individuals are not unlawfully deprived of their liberty. It is often used as a means of protection against unlawful detention by government authorities.

Mandamus (option 1) is a different type of writ that is used to compel a government official or authority to perform a certain action. Certiorari (option 3) is a writ used to review a decision made by a lower court. Prohibition (option 4) is a writ used to prevent a lower court from taking a certain action.