**Core Concept**
The nucleus of an atom is the central part that contains the majority of its mass, consisting of protons and neutrons. This region is responsible for the atom's overall charge and stability. Understanding the composition of the nucleus is crucial in nuclear medicine and radiation therapy.

**Why the Correct Answer is Right**
The nucleus of an atom is composed of protons and neutrons, which are collectively known as nucleons. Protons have a positive charge, while neutrons are neutral. The number of protons in the nucleus determines the element of an atom, whereas the number of neutrons can vary, leading to different isotopes of the same element. The strong nuclear force holds the nucleons together within the nucleus.

**Why Each Wrong Option is Incorrect**
**Option A:** Electrons - These are negatively charged particles that orbit the nucleus, not part of it.
**Option B:** Nucleus contains electrons - As mentioned, electrons are found in the electron cloud surrounding the nucleus, not within it.
**Option C:** Neutrons only - While neutrons are indeed present in the nucleus, it also contains protons, making this option incomplete.

**Clinical Pearl / High-Yield Fact**
In nuclear medicine, radioactive isotopes are used for diagnostic and therapeutic purposes. Understanding the composition of the nucleus is essential for developing and implementing these treatments.

**Correct Answer: C. Neutrons and protons.**