The only option that is not generally a reason why historians study societal change would be "<span>d. to estimate how long it would take to build a road system," since this requires a separate type of research. </span>
South Vietnam needed the US assistance in the early 1960´s because North Vietnam at that time was supported by the Soviet Union, China and other communist countries fighting against South Vietnam with no possibility of containing the communist advance.
After the US withdrew from the conflict, the entire region was taken under the communist government.
The correct answer is option C. East India Company.
The British government intended to help the East India Company with the Tea Act of 1773.
Hope I could help! :)