Answer is "alpha particle only".
alpha decay is an emission of ₂⁴He nucleus. If an element undergoes an alpha decay, the mass number of the daughter nucleus formed is reduced by 4 compared to mass number of parent atom and atomic number is reduced by 2 compared to atomic number of parent atom.
Po has atomic number 84 and mass number as 214. Hence, the formed daughter atom should have 84 - 2 = 82 as the atomic number and 214 - 4 = 210 as the mass number. The decay reaction is
₈₄²¹⁴Po → ₈₂²¹⁰Pb + ₂⁴He