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The correct answer is Option B — 2 to 3 neutrons with an average of 2.5. When U-235 absorbs a slow neutron and undergoes fission it splits into two smaller nuclei (fission fragments) and releases approximately 2 to 3 fast neutrons along with enormous energy (about 200 MeV) and gamma radiation. The average is 2.5 neutrons per fission. These released neutrons when slowed by a moderator can go on to cause further fissions creating a chain reaction. Option A is incorrect: If only 1 neutron were released the chain reaction would barely sustain itself (k=1 at best). Option C is incorrect: U-235 does not release 5 neutrons per fission event. The actual value is approximately 2 to 3. Option D is incorrect: Ten neutrons per fission is far from the actual value of 2.5. The 2.5 average means enough neutrons for chain reaction while still being controllable.
