Which of the following elements can be the fuel in a nuclear fission reaction?

Question

helium
hydrogen
iron
uranium

Answer 1

Uranium. In a fusion reaction it would be deuterium and tritium which are forms of hydrogen.

Answer 2

Uranium only.
Hydrogen and Helium undergo fusion reactions (particularly in the cores of stars, or in fusion (hydrogen) bombs.
Iron does not undergo fusion. In the life of a star, the formation of iron spells the end of a star's fusion activity, because the atomic structure of iron is such that it takes more energy to break an iron atom than is created in a fusion reaction. Only in very large stars, which progress to supernovae, does the formation of elements go beyond iron.

Answer 3

Uranium only (U.235) (Converts to Plutonium).