Before Stanford University researchers discovered that more than half of the study participants (children) watched "adult" TV programs, it happened that D. Adults and children discovered soap opera by westerns, and other types of stories acted out on the radio.
Since this question is asking about the preceding event before the Stanford University research on children watching "adult" TV programs, we can state that adults and children discovered soap operas.
Thus, the correct event that happened before the Stanford University experiment is Option D.
Learn more about answering questions about events at https://brainly.com/question/9068423