Fundamental particles are elementary and indivisible

Steps:

• Define a fundamental particle as one not made of smaller constituents, per the Standard Model of particle physics.
• Examine each option: alpha-particle is a helium nucleus (2 protons + 2 neutrons).
• Electron is a lepton with no substructure.
• Neutron and proton are baryons composed of three quarks each.

Why B is correct:

• The electron is an elementary lepton in the Standard Model, with no known internal structure.

Why the others are wrong:

• A: Alpha-particle is composite, formed by protons and neutrons.
• C: Neutron is composite, made of up and down quarks.
• D: Proton is composite, made of two up quarks and one down quark.

Final answer: B