ANS:C - Cobalt bomb

The Cobalt bomb is considered the most lethal weapon among the options listed. While Atom (fusion) bombs, Hydrogen bombs, and Neutron bombs are all devastating nuclear weapons, the Cobalt bomb, also known as a salted bomb or cobalt thorium G bomb, is designed to produce long-lasting and highly radioactive cobalt-60 isotopes as a significant part of its explosive yield. Cobalt-60 has a long half-life (around 5.27 years) and emits highly penetrating gamma rays, making contaminated areas uninhabitable for extended periods, even decades or centuries. This is due to the intense and persistent radiation it emits, which can cause widespread and long-term harm to living organisms. Due to its ability to create widespread and long-lasting contamination, the Cobalt bomb is often considered one of the most lethal weapons in terms of its long-term effects on both human populations and the environment.