The stability of benzenediazonium salts is governed by the electronic effects of the substituents present on the benzene ring.
The diazonium group is strongly electron-withdrawing. Electron-donating groups (EDG) stabilize the diazonium cation by dispersing the positive charge through resonance or inductive effects, whereas electronwithdrawing groups (EWG) destabilize it by increasing the positive charge density.
In , the group at the para position is a strong electron-donating group via the effect, which significantly stabilizes the diazonium ion.
In , there is no substituent (hydrogen), providing a baseline stability.
In , the group is an electron-withdrawing group via and effects, which destabilizes the cation.
In , the group is a very strong electron-withdrawing group via and effects, even stronger than , leading to the least stability.
Comparing the effects: of of of .

Therefore, the correct stability order is .