The phenomenon explained by tunnel effect is
(A) $\beta$ -decay
(B) $\alpha$ -decay
(C) $\gamma$ -decay
(D) All the above

Hint: The tunnel effect is the name given to the phenomenon that describes the crossing of a quantum mechanical potential barrier by the particle with some energy. An alpha particle is also known as the nucleus of a helium atom.

Complete step-by-step solution:
Inside the nucleus, the alpha particles are present in the potential region inside the nucleus. A radioactive nuclei leads to release of energy in the form of alpha particle decay. The alpha particle comes out of nuclei carrying energies that are very specific. The spectra of energies of the alpha particle obtained from it is a discrete spectra. The discrete spectra arises because the alpha particle comes out of the nucleus carrying the energy that the parent nucleus had. It has been theorized that the energies that the parent nucleus has is in the form of discrete levels of a well just like atomic levels. Therefore, for the alpha particles to come out of the nucleus, the energy equivalent to the difference in the energy levels is required so that the particle that comes out also has that amount of energy.
In tunnelling, a quantum particle with some energy tunnels through the barrier and is able to cross even a potential barrier. Therefore, the alpha particle in the nucleus crosses the potential barrier of the nucleus and comes out.
Therefore, the correct answer is option (B).

Note: Beta decay is the term used to describe the emission of electrons or positrons from the nucleus and gamma decay is the term used to describe the emission of neutral particles from the nucleus.