A neutron is converted into a proton with the emission of an electron and:

The correct answer is Antineutrino. In beta decay, a neutron within an atomic nucleus is converted into a proton, an electron, and an electron antineutrino. This process ensures the conservation of energy and momentum. The reaction is: n → p⁺ + e⁻ + ῡₑ (antineutrino). Among the options:

• Positron: The antiparticle of the electron, emitted in beta‑plus decay (proton → neutron).
• Photon: A particle of light, not emitted in beta decay.
• Antineutrino: Correct; it is emitted along with the electron.
• Alpha Particle: A helium nucleus, emitted in alpha decay.

Hence, the correct answer is "Antineutrino".