Which reaction takes place in a nuclear fission reactor? C+HN
2 Put He → Cm
OCO+ He-Cot 'n
o U+ nKr+Ba+3'n

The reaction that takes place in a nuclear fission reactor is answer D.

Explanation:

Step-by-step explanation:

Nuclear fission reactions involves the process by which large atomic nuclei are bombarded with energetic particles and release of an enormous amount of nuclear energy, which  mainly ends up as heat energy.

So now, consider the nuclear reaction from D, after the uranium nucleus absorbs the neutron its split into two smaller nuclei (often radioactive themselves), Krypton and Barium and releases more neutrons (or energy).