The Bohr model is valid only for single-electron species (hydrogen-like species). The He-4 atom contains two electrons (statement c). In a multi-electron system, in addition to the attraction from the nucleus, each electron experiences repulsive forces from the other electron(s). Consequently, the net force on an electron is not directed solely towards the nucleus, meaning the electrons are not subject to a simple central force field (statement d). This complication renders the simple Bohr model inapplicable.