A. Pauli's exclusion principle: No two electrons in an atom can have the same set of four quantum numbers. Or "Only two electrons may exist in the same orbital and these electrons must have opposite spin.
B. Heisenberg's uncertainty principle: It states that you can never simultaneously know the exact position and the exact speed of an object.

C. Hund's rule of maximum multiplicity: It states that pairing of electrons in the orbitals belonging to the same subshell ( or ) does not take place until each orbital belonging to that subshell has got one electron each with same spin i.e., it is singly occupied.
D Aufbau principle states that electrons first occupy the lowest energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled.
Therefore Heisenberg's uncertainty principle is responsible to rule out the existence of definite paths or trajectories of electrons.