Answer: The correct answer is "There was a slowdown in productivity growth.".
Explanation: The reason behind the slow growth in U.S. incomes during the 1970s and 1980s is that there was a slowdown in productivity growth.
In the decade between 1970 and 1980 the United States went through various economic problems that caused a slowdown in productivity growth which inevitably caused income growth to be noticeably slower.