In this question, we are asked to identify which option is not a writ. A writ is a legal document issued by a court that commands someone to do something or refrain from doing something.

Option 1: Mandamus is a writ that is used to compel a public official or government entity to perform a specific duty that they are legally obligated to perform. It is used to enforce the rights of individuals.

Option 2: Habeas Corpus is a writ that is used to bring a person who is detained or imprisoned before a court to determine the legality of their detention. It is used to protect individual liberty.

Option 3: Certiorari is a writ that is used by a higher court to review the decision of a lower court. It is used to ensure that legal errors are corrected and to maintain consistency in the interpretation of the law.

Option 4: Severability is not a writ. Instead, it is a legal principle that is used to determine if a provision in a law or contract is separate and independent from the rest of the document. If a provision is found to be severable, it can be enforced even if other parts of the document are found to be invalid.

Therefore, the correct answer is option 4, Severability, as it is