Concept:
Thorium undergoes radioactive decay through a series of $\alpha$ and $\beta$ emissions, producing several intermediate radioactive elements.
• Thorium decay series includes elements like radium, radon, and polonium.
• Polonium (Po) is a well-known radioactive decay product in this series.
Step 1: Identify radioactive elements.
Among given options, only polonium is a known radioactive decay product.
Step 2: Conclusion.
Thus, Po is formed during thorium decay.
Final Answer: Option (C)