Final answer:
Consuming a reasonable quantity of alcohol is most likely to cause drowsiness in drivers, as alcohol is a depressant that impacts alertness and coordination, whereas driving with windows open and drinking strong coffee have opposite effects.
Step-by-step explanation:
The thing most likely to cause drowsiness in drivers is consuming a reasonable quantity of alcohol. Alcohol is a depressant that can impair the nervous system and lower a person's alertness, reaction time, and coordination. Despite the legality of consuming small amounts of alcohol, it can still significantly affect one's ability to drive safely. Conversely, driving with windows open may help keep a driver alert by providing fresh air and potentially avoiding the effects of a warm, stuffy vehicle. Drinking strong coffee, containing caffeine, typically acts as a stimulant and may temporarily increase alertness and help ward off drowsiness, making it an unlikely cause of drowsiness in drivers.