I think the question here is how many neurons were released? Because if the question was about the U-235 atoms, the answer would be 3.
In the first stage, 3 neutrons were released. The energy produce from the release of these neutrons would cause another fission. It would yield another 3 neutrons. So, the total neutrons during the 2nd stage is 6. The cycle repeats again on the 3rd stage releasing another 3. Therefore, the total neutrons are 9. The answer is letter B.
