answer: the correct answer is 1.
explanation: there are various process in which a radioactive nuclei decays:
1) alpha decay: in this process, a heavier nuclei decays into lighter nuclei by releasing alpha particle. the mass number is reduced by 4 units.
2) electron capture: in this decay process, a parent nuclei absorbs an electron and gets converted into a neutron. simply, a proton and an electron combines together to form a neutron. mass number does not change in this process.
3) gamma ray emission: it is a decay process in which an unstable nuclei gives excess energy by a spontaneous electromagnetic process. this decay releases 
. this process does not change the mass number.
4) positron emission: it is a type of decay process, in which a proton gets converted to neutron and an electron neutrino. this is also known as 
-decay. in this the mass number remains same.
from the above information, we see that only alpha-decay is producing a change in the mass number of the nucleus.
