Correct Answer: Option B i.e. decreased by four
Reason:
Alpha particle are Helium nucleus (4 2 He). Hence, when an alpha particle is emitted from the radioactive particle, for mass balance, it's atomic number decreases by 2, while atomic mass number decreases by 4. For instance, consider following radioactive disintegration process:
238 92U → 234 90Th + 4 2He
From above example, it can be seen that, upon emission of alpha particle, mass number of 'U' decreased by 4.
