Leptons are fundamental particles including electrons, positrons, and neutrinos

Steps:

• Recall leptons: charged (electron, muon, tau) or neutral (their neutrinos); excludes quarks and composites like protons/neutrons.
• Examine option A: electron (charged lepton), neutrino (neutral lepton), positron (antilepton).
• Examine other options: identify non-leptons (protons/neutrons are baryons made of quarks).
• Confirm only A has exclusively leptons.

Why A is correct:

• All three—electron, neutrino, positron—are fundamental leptons or antileptons, per the Standard Model classification of fermions not interacting via strong force.

Why the others are wrong:

• B: Neutron and proton are baryons (quark composites), not leptons.
• C: Proton and neutron are baryons, not leptons.
• D: Neutron is a baryon, not a lepton.

Final answer: A