In an atom, the order of filling up of the orbitals is governed by

The correct answer is A. The Aufbau principle states that in the ground state of an atom, electrons fill atomic orbitals of the lowest available energy levels first before occupying higher levels. The word Aufbau is German for building up. This principle establishes the specific sequence such as 1s, 2s, 2p, and so on, based on the increasing energy levels of the orbitals. While the other principles are related to atomic structure, they govern different aspects. Paulis exclusion principle states that no two electrons can have the same four quantum numbers. Hunds rule deals with the doubling up of electrons in orbitals of the same energy. Heisenbergs uncertainty principle relates to the impossibility of simultaneously knowing the exact position and momentum of an electron. Only the Aufbau principle directly dictates the sequential order of filling.