The answer is D. Influential
Step-by-step explanation:
In the excerpt from "Finding Unity in the Alabama Coal Mines", the word "powerful" in "forming a powerful interracial labor union" is used to show the reader the strength and importance of the UMWA. Additionally, in this context, the word "powerful" is used as a synonym of "influential" because the author uses this word to explain the effect or impact the organization had regarding civil right movements, even before the first movements emerged "fifty years before the civil rights movement of the 1950s and ’60s". Thus, the connotation or secondary meaning of this word in the text is "influential"