Fundamental particles are elementary, not composite.

Steps:

• Define fundamental particle as one not composed of smaller particles, per the Standard Model of particle physics.
• Examine each option: electron is a lepton; hadron is a category of particles; neutron and proton are baryons.
• Classify: only the electron lacks substructure, while others are built from quarks and gluons.
• Confirm: electron is listed as elementary in particle data groups.

Why A is correct:

• Electron is a fundamental lepton in the Standard Model, with no internal structure.

Why the others are wrong:

• B: Hadron is a composite particle class, including mesons and baryons made of quarks.
• C: Neutron is a composite baryon of three quarks (udd).
• D: Proton is a composite baryon of three quarks (uud).

Final answer: A