All the foursingle neutron /proton in the image coming out of the chain reactions and bombarding are responsible of producing a chain reaction.
What is a nuclear reaction ?
It is the change in the identity or characteristics of an atomic nucleus, induced by bombarding it with an energetic particle.
The bombarding particle may be an alpha particle, a gamma-ray photon, a neutron, a proton, or a heavy ion.
In the given image of a nuclear reaction the objects which could hit other atoms, producing a chain reaction are all the four single neutron /proton coming out of the chain reactions and bombarding.
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