i think the answer is c. an element or compound that cannot be physically separated. sorry if im wrong.
answer: the correct answer is option 2.
radioactive substances are unstable in nature and tend to get stable by some nuclear reactions.
nuclear reaction are the reactions which changes the nucleus of an atom. there are two nuclear reactions: nuclear fission and nuclear fusion.
these two processes can make a radioactive sample can convert it into a non-radioactive sample.
nuclear fission reactions: in these reactions a larger nuclei (parent nuclei) splits into two or more stable daughter nuclei.
nuclear fusion reactions: when two or more smaller nuclei combine to form one larger nuclei, these reactions takes place.
in the question, it is asked the characteristic of a non-radioactive daughter isotope after it is formed from a parent isotope. the parent isotope is undergoing nuclear fission reaction and is forming a stable nucleus.
hence, the correct answer is option 2.