Hadron Definition and Particle Classification

Steps:

• Define hadron as a subatomic particle composed of quarks bound by strong force.
• Identify leptons as fundamental particles not made of quarks.
• Classify electron, neutrino, and positron as leptons.
• Recognize proton as a baryon made of three quarks.

Why D is correct:

• Proton is a hadron because it is a baryon composed of three quarks (two up, one down), following the quark model in quantum chromodynamics.

Why the others are wrong:

• A. Electron is a fundamental lepton, not composed of quarks.
• B. Neutrino is a fundamental lepton that interacts only via weak force.
• C. Positron is the antiparticle of the electron, also a lepton.

Final answer: D