Which best explains why some radioisotopes decay in a decay series? radioactive materials will always be radioactive. multiple decays are always required to achieve stability. some unstable materials decay radioactively into other unstable materials. unstable materials have varying half-lives.

Which best explains why some radioisotopes decay in a decay series?

The correct answer is:
Some unstable materials decay radioactively into other unstable materials.

Radioactive decay a the spontaneous process through which an unstable atomic nucleus breaks into smaller, more stable fragments. It's basically a matter of thermodynamics. Every atom seeks to be as stable as possible. In the case of radioactive decay, instability occurs when there is an imbalance in the number of protons and neutrons in the atomic nucleus.