Which of the following elements is NOT radioactive?

Element 89 - Ac (Actinium) is not radioactive. Actinium is a radioactive element, but it does not have any stable isotopes. All of its isotopes are radioactive, including the most stable isotope Ac-227, which has a half-life of 21.772 years. Therefore, Actinium is a radioactive element and is not the correct answer to the question.
The element 103 - Lr (Lawrencium), element 86 - Rn (Radon), and element 94 - Pu (Plutonium) are all radioactive elements.