Which of the following is not a sign of carbon monoxide presence?

The presence of carbon monoxide (CO) can often be detected through various signs that indicate issues related to combustion appliances or ventilation problems. Among the options, brightly colored flowers are not indicative of carbon monoxide presence because they are not affected by gas concentrations in the air. Flowers generally thrive in environments with adequate sunlight, water, and nutrients, and their coloration is a biological trait rather than a response to airborne toxins like carbon monoxide.

On the other hand, soot on vents, dead plants, and moisture on windows can all be consequences or indicators of problems associated with carbon monoxide. Soot on vents signals incomplete combustion, which can occur when there is insufficient oxygen for burning fuel, leading to carbon monoxide production. Dead plants may indicate poor air quality or overall environmental stress, which could be due to CO or other pollutants. Moisture on windows can be a sign of high humidity levels, which may accompany ventilation issues that also lead to higher CO concentrations.

Therefore, brightly colored flowers stand out as the only option that does not relate to the presence of carbon monoxide or its effects, making it the correct answer in this context.