When a beta particle is released from a nucleus of an atom:

the number of electrons in the atom is increased

the number of protons in the nucleus is increased

the mass number of the nucleus decreases by one

the mass number of the nucleus increases by one

The correct answer is option B. i. e. the number of protons is increased.

In beta-particle emission, a neutron is changed into proton, electron and electron neutrino. The electron is emitted as beta particle and the proton remains inside the atom. Thus, the mass number remains same but the atomic number increases by one.