Final answer:
Electrons are the subatomic particles not found in the nucleus; protons and neutrons, which are called nucleons, reside in the nucleus. The particles which vary in neutron numbers within their nuclei are called isotopes.
Step-by-step explanation:
The subatomic particles that are not found in the nucleus of an atom are electrons. Protons and neutrons are the two types of particles that are present in the nucleus, and they are collectively referred to as nucleons. Electrons, being negatively charged, are found in the electron cloud surrounding the nucleus.
An atom is composed of fundamental subatomic particles: electrons, protons, and neutrons. Neutrons do not interact with other particles through electromagnetic forces as they are neutral, while protons carry a positive charge and electrons carry a negative charge. Other subatomic particles such as quarks make up protons and neutrons, but they are not typically involved in discussions of basic atomic structure.
To answer the multiple-choice question provided: the particles that vary in the number of neutrons in their nuclei are called isotopes (b). Furthermore, regarding neutrinos mentioned in the reference material, they are not hadrons and they lack charge (b).