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.