Answer:
the arrest of an African American by a white police officer
Step-by-step explanation:
According to historical records, it is "the arrest of an African American by a white police officer" that ultimately led to the Watts riots.
This is evident in the fact that on August 11, 1965, an African-American motorist Marquette Frye, was pulled over and arrested by a white police officer, Lee W. Minikus, on California Highway based on being suspected of driving while drunk.