What is the primary function of the gear shift forks inside a motorcycle's manual transmission gearbox?

1. To physically slide the gears along the transmission shafts, engaging the gear dog teeth into the matching slots of adjacent gears.
2. To engage the starting motor bendix gear with the flywheel ring gear.
3. To spray lubricating oil directly onto the transmission main shaft bearings.
4. To measure the rotational speed of the transmission output shaft.