6.D: The aufbau principle
Using the Pauli principle, a third electron cannot occupy a 1s orbital, meaning that we cannot write the wave function for the three electrons of lithium as a product of three 1s single-electron wave functions. Therefore we choose the next highest energy wave function from hydrogen. In hydrogen, the 2s and 2p stationary states had exactly the same energy. In many-electron atoms, the environment each electron feels is close but not identical to hydrogen. More advanced calculations reveal that in all many-electron atoms the 2s states lies slightly below the 2p state in energy. Hence, a 2s hydrogen-like orbital should be written for the third electron of lithium. The lithium wave function given above is abbreviated as ``1s squared,2s''.
The process of placing electrons into hydrogen-like orbitals of successively higher energy is known as the aufbau principle, from the German word which means ``building up''. Use of the aufbau principle is covered in most general chemistry textbooks, where you can continue with a deeper understanding of the underlying physical principles.