What describes an antiparticle?

An antiparticle is defined as a particle that has the same mass as a corresponding particle but possesses opposite charge and other quantum numbers. This means that if you have an electron, which is negatively charged, the corresponding antiparticle, the positron, has the same mass as the electron but is positively charged. Therefore, the identification of an antiparticle focuses primarily on the equality of mass and the inversion of charge.

The emphasis on opposite charge is essential, as it distinguishes antiparticles from regular particles. While each antiparticle matches its particle in terms of mass, other properties such as baryon number, lepton number, and certain quantum states are also reversed.

Other options do not accurately describe the concept of an antiparticle. For instance, saying an antiparticle has an "opposite mass and charge" is misleading because mass cannot be negative; it is always a positive quantity. Similarly, suggesting that an antiparticle is "composed of quark-antiquark pairs" is too specific and only applies to particles like mesons, while “a particle that mediates fundamental forces” refers rather to force-carrying particles such as bosons. Thus, option B provides the correct characteristics that define an antiparticle.