Where did american forces find mass graves containing thousands of south vietnamese who the north vietnamese had executed during the tet offensive?

Hue is the correct answer.

Explanation:

The mass killings of American sympathisers by Viet cong during their withdraw from the city of Hue during the Tet Offensive, is known as Hue massacre. It was one of the longest and bloodiest battles of Vietnam War. The Battle at Hue began on 31st January, 1968 and lasted for twenty six days. After years many mass graves of women, men, infants and children  were discovered around Hue. Five to ten percent of the total population of Hue was killed during the battle of Hue. U.S and South Vietnamese authorities carried out investigation about the massacre and found out about the atrocities done with people.