Here are four statements about radioactivity:
1 A beta particle is an electron that is emitted from the outermost electron shell of
an atom.
2 A beta particle is emitted from the nucleus of the atom when a proton changes
into an electron.
3 A beta particle is emitted from the nucleus of the atom when a neutron changes
into an electron.
4 After a beta particle is emitted the mass number of the nucleus of the atom is
unchanged.
Which statement(s) is/are true?

Answer:

11) D

Remember! A beta particle is a fast moving electron. A neutron turns into a proton and an electron.

The proton stays in the nucleus and the fast moving electron is emitted.

1 – Wrong
The electron comes from the nucleus.

2 – Wrong
A neutron (not a proton) turns into a proton and a electron.

3 – Wrong
A neutron turns into an electron and a proton.

4 – Correct
The mass number is changed. A neutron changes into an electron and a proton. The proton remains in the nucleus. Since the mass number is proton + neutrons, it remains unchanged.

Hence D is correct as only statement 4 is correct.