Answer: c. Americans who sail across the Atlantic on British ships are risking their lives.
Step-by-step explanation:
The answer is indeed option C.
During the first World War of 1914 to 1918, the Allied Nations including Great Britain and the Central Powers including Germany and Austria-Hungary, waged war against each other.
The British being the foremost naval power in the world at the time, implemented a naval blockade of German ports as soon as the war began which led to shortages in Germany. The Germans responded by submarine warfare against the British by originally targeting any British ships sometimes including civilian ships because it was thought that the Brits were transporting munitions on civilian ships.
In response to a query involving any prospective attacks on the Lusitania (a British ocean liner heading back to Britain from the US) which had not left the US at the time, the Germans through their embassy in the USA issued the Notice attached to this question warning that any Americans who sail across the Atlantic on British ships were risking their lives.
The Lusitania was eventually sunk 2 weeks later on May 7, 1915 and 128 Americans drowned showing that the notice was not a mere threat. The sinking drew a sharp outcry against the Germans and increased American support for the Allies.