New neurons in the adult brain arise in the dentate gyrus of the hippocampus. Adult neurogenesis is restricted to two stem-cell niches: the subgranular zone of the hippocampal dentate gyrus, which adds granule neurons that support pattern separation, learning and memory, and the subventricular zone lining the lateral ventricles, whose progeny migrate via the rostral migratory stream into the olfactory bulb. Of the four choices, only the dentate gyrus is one of these niches. The ventral spinal cord does not regenerate neurons; the substantia nigra's dopaminergic cells are not replaced, which is precisely why their progressive loss produces irreversible parkinsonism; and the noradrenergic locus ceruleus is likewise non-regenerative. Therefore the dentate gyrus is the unique correct site.\[\boxed{\text{Dentate gyrus (hippocampus)}}\]