None of the provided options—beta-minus decay, beta-plus decay, gamma decay, or alpha decay—are forms of nuclear fission.
Nuclear fission specifically refers to the process of splitting a heavy atomic nucleus into lighter nuclei, along with the release of energy and additional neutrons. This is distinct from the types of decay you've listed.
Alpha decay involves the emission of an alpha particle (two protons and two neutrons), beta-minus decay involves the conversion of a neutron into a proton and the emission of an electron, beta-plus decay involves the conversion of a proton into a neutron and the emission of a positron, and gamma decay involves the release of gamma radiation from an excited nucleus.
So, the correct answer is that none of these are forms of nuclear fission.