The fundamental force that holds atoms together is the strong nuclear force. This force is responsible for binding protons and neutrons within an atomic nucleus, overcoming the electromagnetic repulsion between positively charged protons. The strong nuclear force operates at very short ranges, typically within the confines of an atomic nucleus, and is significantly stronger than the electromagnetic force at this scale.
While the electromagnetic force does play a role in the structure of atoms by holding electrons in their orbits around the nucleus and influencing chemical bonding, it is not the primary force that binds the nucleus itself. Gravitational forces are relatively weak at the atomic scale and do not contribute significantly to atomic structure. The weak nuclear force is involved in processes like radioactive decay but does not hold atoms together.
Therefore, the strong nuclear force is the correct answer, as it directly correlates with the stability and integrity of atomic nuclei.