Which isotope has the longest half-life?
Half-life reflects nuclear stability. Phosphorus-32 survives about 14 days, sulfur-35 about 87 days, and sodium-24 only 15 hours, making them short-lived tracers for rapid biological processes. Carbon-14, with 5730 years, decays extremely slowly because its neutron-proton ratio is close to stability and its beta decay energy is remarkably low. This exceptional longevity allows survival over geological time, enabling radiocarbon dating and long-term ecological labeling. Among listed choices, carbon-14 retains radioactivity far longer than other common biological isotopes, giving maximum half-life.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.