Question

radioactive materials have unstable
a nuclei.
b electrons.
c electron clouds.
d area of atom outside of nucleus.

Explanation:

Radioactive materials are characterized by having unstable nuclei. Unstable nuclei undergo radioactive decay to become more stable. Electrons, electron clouds, and the area of the atom outside the nucleus are not the source of radioactivity. Radioactivity is a property related to the nucleus of an atom.

Answer:

A. nuclei