The best answer for the question, in which one of the United States’ ally is invaded by another sovereign, would be (C) sending military assistance and troops to defend the UK. This is more likely to happen than just economic sanctions since the UK has close alliance in various contexts with the U.S.
Historically, this has also happened during the World War II, where while maintaining neutrality during the first two years of the war, the U.S also provided military resources such as warships and weaponry to its allies who were involved in the war, which at the time were China, Soviet Union, and United Kingdom.