All of the options you provided played a role in the tumultuous Presidential Election of 1968, but let's analyze each one:
A. The President was assassinated. - President John F. Kennedy was assassinated in 1963, which had lasting effects on American politics, but in the context of the 1968 election, it was the assassination of Robert F. Kennedy during the campaign that significantly impacted the political landscape.
B. Johnson decided not to run for reelection. - President Lyndon B. Johnson announced in March 1968 that he would not seek reelection, which dramatically altered the race and opened the field for other candidates.
C. Nixon was under Congressional investigation. - While Nixon faced various controversies, including allegations related to the Vietnam War, he was not under formal Congressional investigation during the campaign itself. This option is somewhat misleading in the context of the 1968 election.
D. The United States was beginning to be involved in Vietnam. - The Vietnam War had a profound impact on the election, as public opinion was increasingly against the war and it became a central topic in the candidates' campaigns.
The most pivotal factors were A (the assassination of Robert F. Kennedy), B (Johnson not seeking reelection), and D (the involvement in Vietnam). Therefore, A, B, and D are correct, while C may not be as relevant.