AIIMS 2019 Chemistry Quantum Numbers MCQ Question

In the context of electron configuration, the term "m equals to zero" refers to the magnetic quantum number (mₗ), which indicates the orientation of an orbital. For s orbitals, mₗ is always 0, meaning the last electron added for sodium (Na, atomic number 11) is in the 3s subshell, making it an s electron. In contrast, potassium (K, atomic number 19), rubidium (Rb, atomic number 37), and cesium (Cs, atomic number 55) have their last electrons in the 4s, 5s, and 6s subshells, respectively, which do not correspond to the lowest principal quantum number (n=3) where Na's last electron resides. Thus, Na is the only element in the options where the last electron is an s electron in the lowest energy level.