Which one of the following is NOT radioactive ?

Correct answer: Zirconium (4). Zirconium (Z=40) is not inherently radioactive because it lies among the first 82 elements, which each have at least one stable isotope; elements below Z=82 therefore include many stable elements, and zirconium has several stable isotopes [1]. In contrast, astatine (Z=85) and francium (Z=87) are beyond lead (Z=82) and are known to have only unstable (radioactive) isotopes, so they are radioactive [1]. Tritium (hydrogen-3) is a radioactive isotope of hydrogen (a beta emitter) and can be produced as an activation product in reactors [2]. Thus zirconium is the non‑radioactive choice among the options.

Sources

1. [1] https://sciencenotes.org/what-are-the-radioactive-elements/
2. [2] https://www.eesi.org/files/GE_Congress_2021.pdf