Question

In the following nuclear reaction, x stands for:
$n\to p+{{e}^{-}}+x$
$\begin{align}
  & \text{A}\text{. }\alpha \text{ -particle} \\
 & \text{B}\text{. positron} \\
 & \text{C}\text{. neutrino} \\
 & \text{D}\text{. anti-neutrino} \\
\end{align}$

Answer 1

Hint: Neutron decays into a proton by the process of beta decay. Also, the proton decays into a neutron by the same process. Just the difference is that different particles are emitted in the process. Depending on the type of beta decay either neutrino or antineutrino is emitted along with the positron or electron. The total charge in the beta decay is always conserved. Antineutrino is the antiparticle of neutrino and both particles have zero electric charge.

Complete step-by-step answer:
The radioactive decay in which a beta particle is emitted (either electron or positron) is known as beta decay. Conversion of a neutron into a proton and vice versa are the examples of beta decay. By this process of beta decay, an unstable atom can have a stable number of protons and neutrons.
In the conversion of a neutron to a proton, an electron is emitted along with the antineutrino.
The reaction is given as
$n\to p+{{e}^{-}}+{{\bar{v}}_{e}}$
Whereas in the conversion of a proton to a neutron, a positron is emitted with the neutrino.
$p\to n+{{e}^{+}}+{{v}_{e}}$
Thus, in the given reaction x stands for antineutrino.

Additional Information:
There are two types of beta decay: beta plus decay and beta minus decay.
In beta plus decay, a positron is emitted along with the neutrino and in beta minus decay, an electron is emitted along with the antineutrino. Thus, the conversion of a proton to neutron is an example of beta plus decay and the conversion of a neutron to proton is an example of beta minus decay.

Note: Note that protons and neutrons are not fundamental particles. They are made up of quarks. Proton is made up of two up quarks and one down quark whereas neutron is made up of two down quark and one up quark. Beta decay is the evidence that protons and neutrons are not fundamental particles of nature.