Which possession is generally unlawful in the passenger area of a vehicle on a Colorado public road?

Correct answer: An open container of alcohol or marijuana.

Why

Colorado generally prohibits possessing or drinking from an open alcohol container, and using marijuana or possessing an open marijuana container, in a vehicle’s passenger area on a public highway or its right-of-way. For marijuana, the legal definition requires an open or broken seal, partially removed contents, and evidence of consumption in the vehicle. Separate passenger-for-hire and living-area exceptions exist. A sealed item secured outside the passenger area is not the open-container situation in this question.

Verified against Colorado Driver Handbook · Open intoxicant container · p. 6