The purpose of the poster you are referring to may vary depending on its specific imagery and messaging. However, many posters from World War I were created to gain financial support for the war effort through war bonds or to encourage citizens to participate in various home front efforts, like growing victory gardens.
If the poster you mentioned emphasizes financial contributions or saving resources, then the first option is likely correct. If it highlights military enlistment, then it might aim to help mobilize the military.
To provide a definitive answer, I would need more details about the specific content of the poster.