Check every option against the three well known types of radioactive decay one at a time.
Alpha particles come out during alpha decay, so option C is something that is emitted, ruling it out as the answer.
Electrons, as beta particles, come out during beta decay, so option A is emitted too, ruling it out.
Gamma rays are the electromagnetic radiation released during gamma decay, so option B is emitted as well, ruling it out.
Neutrons are not part of alpha, beta or gamma decay at all. A neutron leaves a nucleus only in processes such as nuclear fission, which is separate from natural radioactivity.
After elimination, only neutrons remain as the substance not emitted, so option D is correct.