Answer:
85
Step-by-step explanation:
Beta emission involves the conversion of a neutron to a proton, an electron and a neutrino. This loss of an electron decreases the neutron/proton ratio of the radioactive nucleus.
When radioactive decay occurs via beta decay, the mass number of the daughter nucleus remain unchanged but its nuclear charge increases by one unit. The daughter nucleus is found one place after the parent in the periodic table.
Hence the decay of polonium-84 via beta emission gives the element that is one place after it in the periodic table. Astatine-85.