Fundamental particles are elementary quarks and leptons with no substructure

Steps:

• Recall that fundamental particles in the Standard Model are quarks (6 types) and leptons (electrons, muons, taus, and their neutrinos/antiparticles), excluding composites like protons.
• Check option A: antineutrinos and electrons are leptons (fundamental), but baryons and neutrons are composites of quarks.
• Check option B: electrons, neutrinos, and positrons are leptons (fundamental), but mesons are quark-antiquark composites.
• Check option D: quarks are fundamental; positrons and neutrinos are leptons (fundamental); "leptons" denotes the fundamental class.

Why D is correct:

• Quarks and leptons (including positrons and neutrinos) are the elementary building blocks of matter per the Standard Model, with no subcomponents.

Why the others are wrong:

• A: Baryons and neutrons consist of three quarks each.
• B: Mesons consist of a quark and antiquark.
• C: Hadrons encompass composites like protons (three quarks).

Final answer: D