Fundamental particles are elementary, not composite.

Steps:

• Recall that fundamental particles in the Standard Model are elementary and indivisible, like leptons and quarks.
• Examine each option: electron is a lepton, neutrino is a lepton, neutron is a baryon, top quark is a quark.
• Identify the composite particle: neutron consists of three quarks (up, down, down).
• Confirm: only the neutron is not fundamental.

Why C is correct:

• Neutron is a composite particle made of three quarks, per the quark model in quantum chromodynamics.

Why the others are wrong:

• A: Electron is an elementary lepton with no substructure.
• B: Neutrino is an elementary lepton, one of the lightest fundamental particles.
• D: Top quark is an elementary quark, one of the six fundamental quark flavors.

Final answer: C