The correct answer is option 3: Beta and Gamma radiations only.

The mass number of an element refers to the total number of protons and neutrons in its nucleus. When an element emits radiation, it can undergo different types of decay processes, including alpha decay, beta decay, and gamma decay.

In alpha decay, an alpha particle (which consists of two protons and two neutrons) is emitted from the nucleus of an atom. This means that the mass number of the element decreases by 4, as four particles (two protons and two neutrons) are being lost.

In beta decay, either a beta minus particle (an electron) or a beta plus particle (a positron) is emitted from the nucleus. However, the emission of a beta particle does not affect the number of protons or neutrons in the nucleus, so the mass number of the element remains the same.

In gamma decay, a gamma ray photon is emitted from the nucleus. These photons have no mass or charge, so the mass number of the element is unaffected.

Therefore, if an element only emits beta and gamma radiations, its mass number will not change.