Quark Transformation in β⁻ Decay

Steps:

• Neutron quark composition: one up quark (u) and two down quarks (d), or udd.
• Proton quark composition: two up quarks and one down quark, or uud.
• In β⁻ decay, a down quark (d) in the neutron transforms into an up quark (u) via weak interaction.
• This changes the nucleus's total quarks: down quarks decrease by one, up quarks increase by one.

Why A is correct:

• The transformation d → u directly reduces the total number of down quarks in the nucleus by one, per the standard model of particle physics.

Why the others are wrong:

• B: Up quarks increase by one due to the d → u change.
• C: Down quarks decrease, not increase, from the quark flavor change.
• D: Up quarks increase by one, so they do not stay the same.

Final answer: A