Which of the following gases is primarily colorless and odorless, making it difficult to detect?

Carbon monoxide is primarily colorless and odorless, making it particularly dangerous because it can accumulate in enclosed spaces without being detected by human senses. This characteristic contributes to its potential for causing poisoning, as individuals are often unaware of its presence while it displaces oxygen in the blood, leading to serious health issues or even fatality.

In contrast, oxygen is necessary for human respiration and while it is colorless, it has a distinct role that does not contribute to danger in the same way. Nitrogen dioxide, on the other hand, is known for its reddish-brown color and characteristic pungent odor, making it easier to identify. Carbon dioxide is also colorless and odorless, but it typically has a more recognizable buildup due to its role in respiration and its association with increased levels indicating poor ventilation.