The principle which gives a system to fill the electrons in increasing order of energy level is

The electron should be filled in the orbital in accordance with the increasing order of their energy. This statement is related with Aufbau principle.
According to Aufbau principle (also known as (n+$\mathcal{l}$) rule), electrons are filled in different orbitals from lower (n+$\mathcal{l}$) value orbital to higher (n+$\mathcal{l}$) value orbital.
In case (n+$\mathcal{l}$) value is same, electron is filled first in orbital with lower value of n.