Question

which feature of nuclear fission reactions allows these reactions to take place in a chain reaction?
thermal energy is released and heats up the reactants.
more smaller atoms form that can undergo fission.
a larger atom is formed, which can then be split.
neutrons start the reaction and are released during it.

Explanation:

In nuclear fission chain - reactions, a neutron collides with a large nucleus, causing it to split and release more neutrons. These released neutrons can then go on to cause fission in other nuclei, continuing the chain - reaction. Thermal energy release doesn't directly enable the chain - reaction mechanism. Formation of smaller fissionable atoms isn't the key to the chain - reaction process. In fission, a large atom splits, not a larger atom is formed for further splitting.

Answer:

D. Neutrons start the reaction and are released during it.