Final answer:
The most likely explanation for labor union difficulties since 1980 is the loss of unionized manufacturing jobs to low-wage countries, as part of wider economic changes and the effects of globalization.
Step-by-step explanation:
The question pertains to the difficulties faced by labor unions since 1980. The most likely explanation for these difficulties is A. The loss of once-unionized manufacturing jobs to low-wage countries. This reflects the shift from manufacturing to service industries and the impact of globalization that led to increased competition from foreign producers. The U.S. saw a significant decline in manufacturing jobs, from a peak of 19 million in the late 1970s to 17 million in 2013, while service industry jobs soared from 35 million to over 118 million during the same period. Unions have historically been stronger within the manufacturing sector, and as jobs shifted away to service sectors—where unions have found it more challenging to organize—the proportion of unionized workers has decreased.
Other plausible explanations include the rise of workplace protection laws, which may have reduced the perceived need for unions, and the U.S. legal environment, which can hinder the ability of unions to organize and expand their membership. However, these factors alone do not fully explain the decline, as other developed economies have experienced similar trends without as significant a drop in union membership. Cultural attitudes and national labor policies also play a critical role.