In a radioactive decay of a nucleus, an electron is emitted. This occurs because of the conversion of a neutron into a proton. Option 2 is correct because during this conversion, an electron is created. Neutrons have a slightly higher mass than protons, and during radioactive decay, a neutron can transform into a proton to create a more stable nucleus. To ensure conservation of electric charge, an electron is simultaneously created and emitted. This emitted electron can then be detected as a beta particle in a radiation experiment.

Options 1, 3, and 4 are incorrect explanations. Option 1 states that electrons are present inside a nucleus, which is not true. Electrons are negatively charged particles found outside the nucleus. Option 3 suggests that an electron is created when a proton transforms into a neutron, which is also incorrect. In reality, during this conversion, a positron (a positively charged electron) is emitted. Option 4 mentions conservation of momentum, but the emission of an electron during radioactive decay is primarily driven by the conversion of a neutron into a proton, not momentum conservation.

Therefore, option 2 is the correct explanation for the emission of an electron during radioactive decay.