Which of the following best defines nuclear fusion?
(a) The splitting of a heavy nucleus into two or more lighter nuclei, typically after absorbing a neutron.
(b) The combination of two light nuclei into a single heavier nucleus.
(c) The spontaneous transformation of an unstable nucleus into a different nucleus, usually accompanied by particle or radiation emission.
(d) The joining of two atoms through the sharing of electrons in their outer shells to form a molecule.