Fundamental particles are elementary, not composite.

Steps:

• Recall that fundamental particles include leptons (e.g., electrons, neutrinos), quarks, and their antiparticles; they are not made of smaller particles.
• Examine each option to identify if all listed items are fundamental or include composites like hadrons (baryons, mesons) or nuclei (alpha).
• Eliminate options with any composite particles.
• Select the option where all are fundamental.

Why D is correct:

• Leptons, quarks, and positrons (antiparticles of electrons, a lepton type) are all elementary particles per the Standard Model.

Why the others are wrong:

• A: Meson is composite (quark-antiquark pair).
• B: Baryon is composite (three quarks).
• C: Alpha is composite (helium nucleus).

Final answer: D