Which country is not part of the Golden Triangle (opium-producing area)?

Cambodia in not one of the three countries of the Golden Triangle.

The Golden Triangle is the area where the borders of Thailand, Laos, and Myanmar meet at the confluence of the Ruak and Mekong rivers. The name "Golden Triangle" (coined by the Central Intelligence Agency) is commonly used more broadly to refer to an area of approximately 950,000 square kilometres (367,000 sq mi) that overlaps the mountains of the three adjacent countries.

Along with Afghanistan in the Golden Crescent, it has been one of the largest opium-producing areas of the world, since the 1950s. Most of the world's heroin came from the Golden Triangle until the early 21st century when Afghanistan became the world's largest producer.

Cambodia (officially the Kingdom of Cambodia) is a country located in the southern portion of the Indochina peninsula in Southeast Asia. It is 181,035 square kilometres (69,898 square miles) in area, bordered by Thailand to the northwest, Laos to the northeast, Vietnam to the east and the Gulf of Thailand to the southwest.