Which of these substances can significantly impair driving ability?

Marijuana can significantly impair driving ability due to its effects on the central nervous system. It affects cognitive functions such as attention, time perception, and coordination, which are crucial for safe driving. Specifically, marijuana can slow reaction times and hinder the driver's ability to make quick decisions. Moreover, studies have shown that marijuana can affect motor skills and create an altered sense of time, increasing the likelihood of a crash.

In contrast, substances like water, caffeine, and nicotine, while they may have some effects on the body, do not typically lead to significant impairments that affect driving safety to the extent seen with marijuana. Water is essential for hydration and has no detrimental impact on driving performance; caffeine, while it can lead to overstimulation in excessive amounts, is generally recognized as a mild stimulant that may help with alertness; and nicotine can have stimulating effects as well, but it does not greatly impair driving ability like marijuana does.