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.

The correct answer to the question is -  Neutrons start the reaction and released during it.

EXPLANATION:

Nuclear fission is the type of thermo nuclear reaction in which a heavy unstable nucleus will split into light stable nuclei with the release of energy after absorbing a neutron.

One can be understood it through the decay of uranium nucleus.

Uranium-235 is a heavy unstable nucleus. A moving neutron is projected towards it which will be absorbed by the nucleus . The nucleus will be more unstable, and it will form an intermediate state which undergoes fission. Hence, thermal neutrons are responsible for initiating such type of reaction.

The reaction can be written as follows -

From above, we see that the fission of one uranium nucleus results three extra neutrons which will again initiate the nuclear fission .

Hence, the correct answer to the question will be - Neutrons start the reaction and released during it.