What happens during a β^- decay?
(A) An atomic electron is ejected
(B) An electron, already present within the nucleus, is ejected
(C) A proton in the nucleus decays, emitting an electron
(D) A neutron in the nucleus decays, emitting an electron

Hint: Firstly, we should understand the concept of β-decay. A beta ray is emitted from an atomic nucleus during a radioactive decay process. This process is known as beta decay. The proton in the nucleus transforms from a proton to a neutron during beta decay, and vice versa. Beta decay occurs due to weak nuclear forces. Beta decay is of two types: β⁺ decay and β^- decay.

Complete Step by Step Solution:
β^- decay (negative beta decay) is the type of beta decay in which a negatively charged beta particle, called an electron, is emitted along with an antineutrino.
$n(neutron)\xrightarrow{{}}p(proton)+{{e}^{-}}(electron)+\overline{\upsilon }(antineutrino)$
Hence, during β-decay, a neutron in the nucleus decays by emitting an electron.
Correct Option: (D) A neutron in the nucleus decays, emitting an electron.

Additional Information: β⁺ decay (positive beta decay) is the type of beta decay in which a positively charged beta particle, called a positron, is emitted along with a neutrino.
$p(proton)\xrightarrow{{}}n(neutron)+{{e}^{+}}(positron)+\upsilon (neutrino)$
Beta decay is important to maintain the proton to neutron ratio by the emission of radiation, which increases the stability of the nucleus.

Note: We should not get confused between β⁺ decay and β^- decay. These are opposite to each other. When the number of neutrons is low, then β⁺ decay occurs. whereas, if the number of protons is low, then β^- decay occurs.