Answer: Weak.
Step-by-step explanation:
The complete question is:
Scientists use a substance called Iodine-131 to treat cancer. As Iodine-131 undergoes radioactive decay and emits beta particles, it kill cancer cells. Which fundamental force is being used in this type of treatment?
Well, here we have beta decay.
Beta-decay is when a beta particle (an electron or a positron) is emitted from an atomic nucleus (usually a neutron-rich nucleus). We know that beta decay is a consequence of the weak force or weak interaction (the force responsible for the radioactive decay of atoms)
Then the correct option is the second one, weak.