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