Which type of censorship should be employed (No censorship, informal or formal) when information may be considered a threat to national security?
Informal censorship would be the best kind of censorship because it would allow national security to be protected without having to go through the messy and perhaps dangerous process of trying to set clear, legal limits on what information may or may not be disseminated.
The best way to do this would be to have the government inform media as to what news should not be disseminated. The media would then voluntarily refrain from publishing or broadcasting that information. This would be much simpler than trying to write rules that would spell out what sorts of information should and should not be printed. It would also be much less dangerous to national security than a system in which there was no censorship. It would also be less dangerous than having a system of rules where sensitive information might be able to slip through loopholes in the rules.
The best system, then, would be an informal one in which media cooperates with government and does not publish or broadcast sensitive information.