An operator should get out of a manhole as soon as s/he feels:

The correct answer focuses on the importance of safety and awareness of one's physical condition while working in potentially hazardous environments such as manholes. Feeling drowsy is a significant indicator of fatigue or a potential lack of oxygen, both of which can pose real dangers when working in confined spaces. An operator experiencing drowsiness may not be alert enough to recognize further escalating dangers, such as changes in air quality or the development of harmful gas concentrations.

In confined spaces, like manholes, the risk of hazardous atmospheres increases, making it crucial to respond to signs of impaired cognitive function or fatigue promptly. Drowsiness can also reduce reaction times, which is critical when safety is a concern.

While dizziness and lightheadedness are also warning signs that should prompt caution, they may not necessarily indicate a state of impaired alertness as clearly as drowsiness does. Feeling colder than normal could indicate issues with temperature regulation but does not directly point to cognitive or alertness concerns that would necessitate an immediate exit. Therefore, the emphasis on drowsiness aligns with best safety practices for maintaining awareness and taking appropriate action in potentially dangerous situations.