Unstable atoms usually are very large atoms with a big nucleus, this configuration sometimes gives the atoms this unstable characteristic, which is why they decay so fast (some will decay in a matter of milliseconds), and radioactive decay is a way that atoms can find to become more stable, in this type of process, particles (alpha and beta decay) and energy (gamma decay), can be emitted from an atom in order to reach stability, therefore the best option is the 3rd option
