One example of decay series is the sequence of decays that starts with Thorium-232 which ends up as Lead-208.
What does it mean?
We mean that Thorium-232 decays into Radium-228 plus and an alpha particle. Then, Radium subsequently decays into Actinium-228 via beta decay, and so on until Lead-208 is the final end product.
Answer: Radium-228