which of the following cannot be emitted in radioactive decay of the substance? (A) Helium-nucleus (B) Electrons (C) Neutrinos (D) Proton.

Helium-nucleus, also known as an alpha particle, consists of 2 protons and 2 neutrons. It is a common particle emitted during radioactive decay, especially in heavy elements like uranium or plutonium. An unstable nucleus emits an alpha particle to reduce its mass and become more stable. So, a Helium-nucleus can be emitted in radioactive decay.

Electrons can be emitted during a type of radioactive decay known as beta decay. In beta-minus decay, a neutron in the nucleus transforms into a proton, releasing an electron (beta particle) and an electron-antineutrino. In beta-plus decay or electron capture, a proton in the nucleus transforms into a neutron, releasing a positron (anti-electron) and an electron-neutrino. Therefore, electrons can also be emitted in radioactive decay.

Neutrinos are elusive, nearly massless particles that travel at nearly the speed of light. They are emitted during beta decay, as mentioned previously. In beta-minus decay, an electron-antineutrino is released alongside the electron. In beta-plus decay, an electron-neutrino is released along with the positron. Thus, neutrinos can be emitted in radioactive decay.

Protons are positively charged particles found in the nucleus of an atom. Generally, protons are not emitted alone during radioactive decay processes, such as alpha or beta decay. In fact, the only known process for a single proton to be emitted from a nucleus is known as proton emission. However, proton emission is a rare type of radioactive decay that only occurs in a few specific conditions, such as in extremely proton-rich isotopes. In comparison to alpha, beta, and gamma decay, proton emission is not a common process. Thus, (D) Proton is the correct answer, as it is rarely emitted in radioactive decay. In conclusion, the particle that cannot be emitted in radioactive decay of the substance is the Proton (D).