During a negative beta decay.

A neutron in the nucleus decays emitting an electron.

In a beta minus decay process, a neutron inside the nucleus converts into a proton and an electron are created.

¹⁴₆C → ¹⁴₆N + e ^-

In the above example a neutron converted into a proton, thus the mass number wasn't altered but the atomic number was increased by one.

Hence,the correct option is (C). ( A neutron in the nucleus decays emitting an electron )