Answer: The correct answer is Option C.
Step-by-step explanation:
Emission of beta particle occurs when an atom undergoes beta decay process.
Beta decay is defined as the process in which beta particle is emitted. In this process, a neutron gets converted to a proton and an electron. The electron released is known as beta particle.
The equation representing beta decay process follows:

From the above equation, the mass number of the nucleus formed does not get changed. The mass number remains the same which is 'A'
Thus, the correct answer is Option C.