According to Hund's Rule,the arrangement of electrons is such that each degenerate (of same energy) orbital is filled if the electrons are available i-e if you have to put electrons in orbitals of subshell 2p and you have four electrons to fill rather than putting 2 electrons in 2px and 2 electrons in 2py and leaving the 2pz orbital empty,you will place one electron in 2px,one in 2py and one in 2pz and the last electron which is left can be paired in any orbital.It is your choice whether you want to put it in 2pz or 2px or 2py.But keep it in mind,in pairing you have to put the electrons with opposite spin.
The error in the question is that they have made the pairs of electron before filling each orbital that is they should place one electron in 2px then 2py,as no electron is now left leave the 2pz orbital empty instead of making the pairs in 2px or 2py or 2pz.
Note:we make pairs in orbitals when each of the orbitals is filled with one electron amd we have left with extra electrons to place!
