Answer
(a) Cs-139
(b) Fe-52
Work Step by Step
The isotope with mass number close to the molar mass of the element is more stable and therefore it has longer half-life.
(a) Molar mass of Cs is $132.9\,u$. The isotope with mass number close to $132.9$ is Cs-139. Therefore, longer half-life is expected for Cs-139.
(b) Molar mass of Fe is $55.845\,u$. The isotope with mass number close to $55.845$ is Fe-52. Therefore, Fe-52 is expected to have a longer half-life.
