The U.S invasion of Cambodia, is the correct option.
Kent State University protest was the protest by students of Kent State University against Richard Nixon's announcement to bomb Cambodia, which was neutral during the Vietnam war. This protest turned violent when Ohio National Guard members opened fire at them. The other important reason was the revelations about President Lyndon Johnson, that he misled the country about the Gulf of Tonkin Incident and the increasing number of US soldiers who were dying in the war. The United States public was divided about the Vietnam issue.
