According to the Washington Driver Guide, the most frequently detected psychoactive substance among drivers is

Answer: A — Alcohol

In Washington, alcohol is the most frequently detected psychoactive substance among drivers. This is because alcohol impairs a driver's ability to operate a vehicle safely by affecting coordination, judgment, and reaction time. Driving under the influence of alcohol is a serious offense and can lead to accidents, injuries, and even fatalities. It is important for drivers to understand the dangers of driving under the influence of alcohol and to always designate a sober driver or use alternative transportation if they have been drinking.