Which of the following is the most stable atom?

Ordinary water is not used as a moderator in nuclear reactors because

${}_{90}Th^{232} \rightarrow {}_{82}Pb^{208}$. The number of $\alpha$ and $\beta$-particles emitted during the above reaction is:

Radioactive isotopes that have an excessive neutron/proton ratio generally exhibit

If the mass defect of ${ }_{5}^{11}B$ is $0.081 \ u$, its average binding energy (in $MeV$) is

Who observed that when the nucleus of a uranium atom was bombarded with fast-moving neutrons,it became so unstable that it immediately broke into two nuclei of nearly equal mass,besides other fragments?