Final answer:
The last discovered subatomic particle in an atom, according to the question's options, is the quark. Quarks make up protons and neutrons, known as nucleons, in the nucleus of the atom. The discovery was made by scattering high-energy electrons off of protons, revealing nearly direct evidence of quarks.
Step-by-step explanation:
The last particle of an atom to be discovered, as per the options provided, is the quark. Quarks are fundamental constituents of matter that combine to form protons and neutrons, which are collectively known as nucleons. These nucleons make up the nucleus of the atom and are responsible for almost all of the atom's mass. In the original discovery experiments, high-energy electrons were launched into protons, which are one type of nucleon, and the behavior of scattered electrons provided nearly direct evidence for the existence of quarks.
Quarks were not discovered in electrons since electrons are a different type of fundamental particle known as leptons. Instead, the discovery of quarks came through experiments involving protons. Specifically, when scientists scattered high-energy electrons off of protons, some electrons were observed to scatter straight backward, pointing to the presence of a very small, dense component inside the protons, which was eventually identified as the quark.