Beta particles are electrons with a specific rest mass

Steps:

• Identify beta particle as a high-energy electron emitted in beta decay.
• Recall rest mass of electron is 9.11 × 10^{-31} kg.
• Note proton/neutron mass is approximately 1.67 × 10^{-27} kg, unrelated to beta particles.
• Match electron mass to options A and B.

Why B is correct:

• Beta particle defined as electron; its rest mass is 9.11 × 10^{-31} kg per standard particle physics data.

Why the others are wrong:

• A: Identical to B but listed as separate option, likely erroneous; does not change correctness.
• C: Matches proton rest mass, not electron.
• D: Identical to C, proton mass irrelevant to beta particle.

Final answer: B