Which of the actinoids is used in the treatment of cancer?

The correct answer is option 4. Thorium.

Out of the listed actinoids, Thorium (Th) is the one used in the treatment of cancer.

Here's a breakdown of the other options:

Plutonium (Pu): This radioactive element is primarily used in nuclear weapons and has no known medical applications.

Uranium (U): While some isotopes of uranium have been explored for potential medical uses, they are not typically used in cancer treatment due to their high radioactivity.

Curium (Cm): This radioactive element also has no known medical applications.

It's important to note that Thorium's use in cancer treatment is currently limited and involves ongoing research. It is not a widely used or established treatment option yet. Additionally, thorium and its compounds can be radioactive and require careful handling due to potential health risks.