Which legal tool is used to compel a witness to testify?

The correct answer is the legal tool known as a subpoena. A subpoena is a formal document that orders an individual to appear and testify as a witness in a legal proceeding or to produce documents relevant to the case. This compelling nature of a subpoena is critical in ensuring that witnesses cannot evade their obligation to testify, which is vital for the pursuit of justice and the integrity of the legal process.

While a summons is typically used to notify a defendant of a legal action and requires them to appear in court, it does not compel witness testimony. A deposition involves the process of taking sworn testimony outside of court, but it does not serve as a command for attendance in the same way a subpoena does. An affidavit, on the other hand, is a written statement made under oath and can be used as evidence, but it does not compel a witness to appear or testify in court. Therefore, the subpoena is specifically designed for compelling witness testimony in the legal context.