Answer: B. A long half-life means the activity of a radioactive sample decreases slowly, so some fission products can remain radioactive over long periods. Secure long-term containment reduces exposure to radiation and prevents radioactive material from reaching the environment.

A: confuses shielding, which reduces radiation reaching people, with changing unstable nuclei into stable nuclei. C: assumes that stopping fission also stops radioactive decay; fission products continue to decay after removal. D: treats a half-life as a fixed expiry time for every nucleus; after one half-life, about half the radioactive nuclei are expected to have decayed.

Remember: shielding reduces exposure; it does not stop radioactive decay.