Final answer:
Atomic physics mainly deals with phenomena involving electrons, while nuclear physics investigates the nuclei of atoms, including protons and neutrons which are composed of quarks (A).
Step-by-step explanation:
Atomic physics deals mainly with the phenomena involving electrons. It is the study of atomic properties and behaviors that arise mostly due to the electron configurations surrounding the nucleus. While protons and neutrons, which are composed of quarks, do play a role in the atomic structure, their study primarily falls under nuclear physics. Nuclear physics explores the substructures of the nuclei, which include the interactions of protons and neutrons. Quarks were originally discovered through experiments that scattered high-energy electrons off of protons.
And indeed, it was the analysis of such scattering experiments that provided nearly direct evidence of quarks. On the other hand, electrons are the primary focus of atomic physics for understanding atomic orbitals, ionization energies, and many other atomic-scale phenomena.