Question

This world war ii poster is suggesting that

Answer 1

Probably the poster you have in view is suggesting that freedom of speech might have its limits in times of war.  
During World War II, there were posters with slogans such as, "Pipe down, soldier! Berlin is listening!"  and "Keep your trap shut! Careless talk may cost American lives!" and "Walls have ears; be careful of what you say."   If the poster your assignment included doesn't have an image or theme about keeping quiet about matters of national security, then maybe add an image of the poster here so we can see what other type of poster you may have in view.