Answer;
Polonium-214
Explanation;
β-decay is the decay of a neutron into a proton. This keeps the mass number the same, since neutrons and protons weigh approximately the same, though increases the atomic number by one, forming a new element.
The bismuth isotope in question has a mass of 214, and the atomic number of bismuth is 83, which we can tell from a periodic table. If the mass number is kept the same and the atomic number increases by one, β-decay of bismuth-214 forms polonium-214 (atomic number 84).
