In an atomic nucleus, neutrons and protons are held together by exchange forces. These forces are also known as nuclear forces or strong forces. Let's explore this concept in detail.

1. **Atomic Nucleus:** The atomic nucleus is the central part of an atom that contains protons and neutrons. Protons have a positive charge, while neutrons have no charge. The number of protons in the nucleus determines the element, while the total number of protons and neutrons determines the atomic mass.

2. **Gravitational Forces:** Gravitational forces are attractive forces between objects with mass. However, the gravitational force is relatively weak when compared to other fundamental forces, such as the strong nuclear force. In an atomic nucleus, the gravitational force is negligible compared to the other forces involved.

3. **Coulombic Forces:** Coulombic forces, also known as electrostatic forces, are attractive or repulsive forces between charged particles. Protons in the nucleus have a positive charge and would repel each other due to these Coulombic forces. However, the nucleus remains stable, which indicates the presence of another force that overcomes the Coulombic repulsion.

4. **Magnetic Forces:** Magnetic forces arise due to the movement of charged particles. While there are magnetic interactions in atomic nuclei, they are not the primary forces responsible for holding protons and neutrons together.

5. **Exchange Forces (Strong Nuclear Forces):** The exchange forces, also known as strong nuclear forces, are the primary forces responsible for holding neutrons and protons together in the atomic nucleus. These forces are fundamentally different from gravitational, Coulombic, and magnetic forces. Exchange forces are attractive forces that act between nucleons (protons and neutrons) and are mediated by particles called mesons.

6. **Strong Nuclear Force:** The strong nuclear force is a short-range force that binds nucleons together. It is stronger than the Coulombic repulsion between protons and acts between all nucleons, irrespective of their charge. This force is responsible for the stability of atomic nuclei.

7. **Mesons:** Mesons are particles that mediate the strong nuclear force. There are different types of mesons, such as pions, that are exchanged between nucleons, creating an attractive force. The exchange of mesons between nucleons creates a strong binding force that overcomes the Coulombic repulsion between protons.

In conclusion, the correct answer is option 'B' - exchange forces. These forces, also known as strong nuclear forces, are responsible for holding neutrons and protons together in the atomic nucleus. They are fundamentally different from gravitational, Coulombic, and magnetic forces and are mediated by mesons.