Answer
The correct answer is:
(b) an electron
Work Step by Step
The upper number is the total number of nucleons (neutrons and protons), while the bottom number is the number of protons.
If $^{215}_{83}Bi$ decays into $^{215}_{84}Po$, then a neutron must have decayed into a proton. In this kind of decay, an electron is also released. We can see that the release of an electron conserves the charge.
The correct answer is:
(b) an electron
