When starting a vehicle with an automatic transmission, what must the gear selector be set in?

The gear selector must be set to (P)ark when starting a vehicle with an automatic transmission because this position locks the transmission and prevents the vehicle from rolling. When the gear is in park, the engine can be started safely without the risk of the vehicle moving unexpectedly. The Park position engages a mechanism that keeps the transmission in a neutral state, allowing the engine to turn over and start without applying power to the wheels.

If the gear selector were set to Neutral, although the engine could start, it wouldn't provide the same level of safety as Park. Starting in Neutral may cause the vehicle to roll if it's on an incline. First and Reverse positions are not suitable for starting because these gears are intended for driving and would cause the vehicle to move immediately after the engine starts, posing a danger to the driver and those around. Therefore, setting the gear selector to Park ensures a safe and secure start to the vehicle.