Which of the following must you do before leaving your parked vehicle?

Before leaving your parked vehicle, it is crucial to effectively use the parking brake along with the gears to secure the vehicle in place. Engaging the parking brake prevents the vehicle from rolling, which is especially important on inclines or if the ground is uneven. Additionally, placing the vehicle in gear (for manual transmissions) or in 'park' (for automatic transmissions) provides an extra layer of security against unintended movement.

Ensuring that the vehicle is secure before exiting not only protects your property but also enhances safety for pedestrians and other vehicles in the vicinity. This practice helps mitigate the risk of accidents, making it an essential habit for every driver.