Mesons consist of a quark-antiquark pair

Steps:

• Recall that mesons are a type of hadron, subatomic particles held together by the strong nuclear force.
• Hadrons are classified into baryons and mesons based on quark content.
• Baryons contain three quarks, while mesons contain a quark and an antiquark.
• Confirm via particle physics: mesons have zero baryon number, matching one quark (+1/3) and one antiquark (-1/3).

Why A is correct:

• By definition in the quark model, mesons are bound states of one quark and one antiquark, satisfying their integer spin and mass properties.

Why the others are wrong:

• B: Two antiquarks with one quark would have negative baryon number, not matching mesons.
• C: Two quarks and one antiquark yield a net baryon number of +1/3, resembling unstable pentaquarks, not mesons.
• D: Three quarks define baryons like protons, not mesons.

Final answer: A