Which of the following violates Hund's rule?
Hund's rule states that pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied. In the third case, third orbital of p is not filled and there are two electrons in the first orbital of p. Therefore, this violates Hund's rule.