Fundamental particles are elementary and indivisible

Steps:

• Define fundamental particles as those without internal structure, per the Standard Model of particle physics.
• Examine each option: electrons and positrons are leptons; neutrinos are also leptons; neutrons are baryons.
• Identify composites: neutrons consist of quarks and gluons, unlike the others.
• Confirm: only the neutron is not elementary.

Why C is correct:

• Neutrons are composite particles made of three quarks (up, down, down) bound by the strong force, as described by quantum chromodynamics.

Why the others are wrong:

• A: Electrons are fundamental leptons with no substructure.
• B: Neutrinos are fundamental leptons that interact only via the weak force.
• D: Positrons are the antiparticles of electrons and thus also fundamental.

Final answer: C