Quark and Antiquark Electric Charges

Steps:

• Quarks are fundamental particles with fractional electric charges in units of e (elementary charge).
• The down quark (d) carries a charge of -1/3 e, as defined in the Standard Model.
• Antiquarks have charges opposite to their quark counterparts.
• Thus, the anti-down quark ($\bar{d}$) has a charge of +1/3 e.

Why B is correct:

• It accurately states the Standard Model charges: -1/3 e for down quark and +1/3 e for anti-down quark.

Why the others are wrong:

• A: Incorrectly assigns positive charge to down quark and negative to anti-down.
• C: Gives positive charge to both, ignoring the negative charge of down quark.
• D: Gives negative charge to both, ignoring the positive charge of anti-down quark.

Final answer: B