The correct answer is D) had trouble finding attackers in the dense jungles.
The US involvement in Vietnam was unsuccessful due to the terrain and tactics used by the Viet Cong (aka Northern Vietnamese military). The use of guerilla warfare and surprise attacks caught Americans off guard numerous times, as the jungles of this country made it difficult to see the enemy coming. The lack of knowledge by many US soldiers made this an uphill battle throughout the US participation in the Vietnam War.