Which of the following elements is fissile?

Plutonium-239 is fissile, sustaining a chain reaction with slow thermal neutrons.

Thorium-232 (A) is fertile, converting to fissile Uranium-233 after neutron capture.

Uranium-237 (C) is an unstable isotope, not a usable fissile fuel.

Uranium-238 (D) is fertile, breeding into Plutonium-239 inside reactors.

Uranium-235 is the only naturally occurring fissile isotope.

Fissile nuclei split on thermal neutrons, whereas fertile nuclei first absorb a neutron to become fissile.