I'm going to assume that the world that this scenario is statist. (or similar to our current world)
Only two of the choices provided are really choices, the other two options are without backing.
Starting with the poor choices
- Retaliate with nuclear weapons
If the leader of said country is killed and these letters are placed into action, the one killed is unable to give input. The one who is killed in this situation is one of the very few people qualified to make such decisions and they can not be properly made postmortem.
- Use his own judgement
The use of under-qualified judgement is a poor decision. Effects of said action will not be properly considered by those who do not have the resources (assuming military personnel) to conduct a proper study into what would happen afterward. Nuclear war isn't an instant, it lasts for generations afterward.
The other two choices are options, but one (don't retaliate with nuclear weapons) will go with the other (change command). The changing of command is the choice with the best possible outcomes. The change of command allows for proper studying into any effects before drastic measures are taken and allows for those who are under-qualified to receive an educated order to fulfill.
