13. A nuclear decay chain represents the reason why only fission is more efficient than fusion for nuclear energy
a series of
decays producing sequentially more stable nuclei
b. the only possible type of decay that an isotope can undergo d. the process of nuclei becoming
increasingly energetic as they release radiation

a series of  decays producing sequentially more stable nuclei

Explanation:

A nuclear decay chain represents a series of decays producing sequentially more stable nuclei.

For every atomic nucleus, there is a specific neutron/proton ratio which ensures the stability of the nucleus. Any nucleus with a neutron/proton combination different from its stability ratio (i.e either too many neutrons or too many protons) will be unstable and split into one or more other nuclei with the attendant emission of small particles of matter. The decay process is a continuous chain reaction steps. Until stability of the nucleus is attain, it continues.

The center is basically the heart of it