Which of the following is a discovery tool used to tell the truth?

The correct choice is voir dire. This term refers to the process during jury selection in which potential jurors are questioned to determine their suitability for serving on a jury. The purpose of voir dire is to uncover any biases, prejudices, or preconceptions that jurors may hold that could affect their ability to deliver an impartial verdict. This questioning process helps ensure that the jury can assess the case based solely on the evidence presented, thereby facilitating a truthful and fair trial.

In contrast, the other options do not serve the same purpose. A waiver refers to the relinquishment of a right or privilege and does not inherently involve discovering the truth. A true bill is a term that pertains to a grand jury's decision to indict someone, which is not a discovery tool in the context of revealing truth during trial proceedings. Vacate generally means to annul or cancel something, like a court decision or order, and does not relate to the process of discovering truth in legal proceedings.