To determine for which atom the Bohr model is not valid, we need to understand the assumptions and limitations of the Bohr model.$\\$The Bohr model is based on the following assumptions:$\\$• It is applicable to single-electron systems, such as hydrogen-like atoms.$\\$• It assumes circular orbits of electrons around the nucleus.$\\$• It does not account for electron-electron interactions, which are present in multi-electron atoms.$\\$Let's analyze each option:$\\$1. Singly ionised helium atom (He$^{+}): \\$• This is a hydrogen-like ion with only one electron, so the Bohr model is valid.$\\$2. Deuteron atom:$\\$• A deuteron is a nucleus of deuterium, which is an isotope of hydrogen. It has one proton and one neutron, but the Bohr model applies to the electron orbiting the nucleus. Thus, it is valid for hydrogen-like atoms.$\\$3. Singly ionised neon atom (Ne$^{+}): \\$• Neon typically has 10 electrons. Even when singly ionised, it still has 9 electrons, which means it is not a single-electron system. The Bohr model is not valid here due to electron-electron interactions.$\\$4. Hydrogen atom:$\\$• The Bohr model was originally developed for the hydrogen atom, which has only one electron. Thus, it is valid.$\\$Therefore, the Bohr model is not valid for the singly ionised neon atom (Ne^{+}), which corresponds to Option 3.}