Answer:
Generally with successive half-life a new element is formed from the decayed nuclei:
isotope - daughter nucleus has same no. of protons (same atomic number) so the same element is formed in the decay
isotone - daughter nucleus has same number of neutrons so the atomic number has changed and a new element is formed (new atomic number)
isobar - daughter nucleus has same mass number - this could be an example of beta decay where the decayed nucleus has the same mass number but loses a neutron and gains a proton because of the lost electron and a new element is formed with the atomic number increases by 1
Example:
U238 Uranium - alpha to Th234
Th234 Thorium - 2 beta to U234 (back to U but atomic mass less by 4)
U234 Uranium - alpha to Th230
Th230 Thorium - alpha to Ra226
etc. but during beta decay (electron loss) a neutron is lost and a proton gained (answer a)
