Final answer:
The process where one atom splits into two or more smaller atoms is called fission, which releases significant energy and is used in nuclear reactors.
Step-by-step explanation:
The process in which one atom splits into two or more smaller atoms is known as fission. Option B) Fission appropriately describes this nuclear reaction. In fission, a large nucleus like U-235 absorbs a neutron, becomes unstable, and splits into two or more smaller atoms, a process that releases a significant amount of energy. This process is different from fusion, in which smaller nuclei join together, and sublimation or ionization, which involve phase changes and the removal of electrons, respectively.