Fission and fusion reactions both involve making changes to an atom's a. nuclei b. electron shells c. electron arrangements d. location for its protons and neutrons

Option (A) is correct. Fission and fusion reactions both involve making changes to an atom's nuclei.

Nuclear fission is a process in which a bigger nucleus breaks into two or more smaller nuclei with the liberation of large amount of energy.

Nuclear fusion is a process in which two smaller nuclei fuse together to form a bigger nucleus. A tremendous amount of energy is released during fusion.

Thus both fission and fusion involve the changes in the nuclei of atoms. Rest of the atom like the electronic configuration etc remains same.