Final answer:
The Watergate scandal was the primary reason for the trends in public confidence from 1973 to 1976.
Step-by-step explanation:
The primary reason for the trends in public confidence from 1973 to 1976 was the Watergate scandal. The Watergate scandal refers to a series of events surrounding the break-in at the Democratic National Committee headquarters in 1972, which eventually led to the resignation of President Richard Nixon.
The scandal had a significant impact on public trust and confidence in the government, particularly in Congress and the Executive Office. The revelations of corruption and abuse of power eroded public confidence and led to a decline in the percentage of people indicating a 'Great Deal of Confidence' in the government.
Other events mentioned in the options, such as the normalization of relations with China, the nuclear-power accident at Three Mile Island, and the explosion aboard Apollo 13, were important events during that time period but they were not the primary reasons for the trends in public confidence.
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