Antiparticles share mass but have opposite charge

Steps:

• Identify the antiproton as the antiparticle of the proton.
• Antiparticles have identical mass to their corresponding particles.
• Antiparticles have opposite electric charge.
• Thus, antiproton has mass $m_p$ and charge $-e$.

Why D is correct:

• Per particle physics definition, antiparticles conserve mass magnitude while inverting quantum numbers like charge.

Why the others are wrong:

• A: Mass is scalar and positive; negative mass violates physical principles.
• B: Mass cannot be negative, as it represents inertial property independent of charge.
• C: Charge must be opposite for antiparticles, per Dirac's theory and CPT invariance.

Final answer: D