In which process is energy typically absorbed rather than released?

In nuclear fusion, energy is absorbed because it involves combining light atomic nuclei to form a heavier nucleus. This process requires an input of energy to overcome the electrostatic repulsion between positively charged nuclei. When fusion occurs, the mass of the resulting nucleus is less than the sum of its parts due to the conversion of mass into energy according to Einstein's equation, E=mc². While fusion can release a significant amount of energy in certain conditions, the initial process itself requires energy to initiate and maintain the fusion reaction, such as the high temperatures and pressures found in stars.

Other processes like nuclear fission, ionization, and alpha decay typically involve energy release. For instance, nuclear fission splits heavy nuclei into lighter ones, releasing energy due to the binding energy difference before and after the reaction. Ionization involves removing an electron from an atom, which usually requires energy input, but is fundamentally an energy-releasing process in many chemical reactions. In alpha decay, a heavy nucleus emits an alpha particle, releasing energy as it completes its transformation into a more stable configuration. Thus, nuclear fusion is distinct in that its initiation requires energy, making it the correct choice.