I think it would be D. Here's why.
"The Invasion of Cambodia. On April 30th of 1970, President Richard Nixon declared to a television audience that the American military troops, accompanied by the South Vietnamese People's Army, were to invade Cambodia. The invasion was under the pretext of disrupting the North Vietnamese supply lines."
Source: https://vietnamawbb.weebly.com/invasion-of-cambodia.html
Hope this helps :)
