Question

the most used element in nuclear fission reactors is which of the following?
thorium
uranium
plutonium
radium

Explanation:

Uranium, specifically the isotope Uranium-235, is the most widely used fuel for nuclear fission reactors because it readily undergoes controlled fission, releasing large amounts of energy that can be harnessed for power. Thorium, plutonium, and radium are not the primary elements used in standard commercial nuclear fission reactors.

Answer:

Uranium