Answer:
The event that directly contributed to Congress passing the War Powers Act of 1973 was:
B. the Gulf of Tonkin incident
The Gulf of Tonkin incident in 1964 played a significant role in escalating U.S. involvement in the Vietnam War. It involved reported attacks on American naval vessels by North Vietnamese forces, which led to the Gulf of Tonkin Resolution that granted President Lyndon B. Johnson broad war powers. The later passage of the War Powers Act of 1973 was partly a response to the perceived executive overreach in committing U.S. forces to military actions without explicit Congressional approval, as exemplified by the Gulf of Tonkin incident.