Here's why:

* Quarks are the fundamental building blocks of protons and neutrons. Protons and neutrons, in turn, make up the nucleus of an atom.

* The strong force is responsible for binding quarks together to form protons and neutrons. It is the strongest of the four fundamental forces in nature.

* The strong force also holds protons and neutrons together within the nucleus. This is what keeps the nucleus stable and prevents it from flying apart due to the electrostatic repulsion between protons.

Therefore, the strong nuclear force directly acts on quarks, the fundamental particles that make up protons and neutrons.