The correct answer is option 4, Traction load. Traction load refers to the movement of heavy rocks and sediments along the river bed through rolling or sliding. In river transport, the heaviest rocks are not lifted or suspended in the water column, nor do they jump or bounce along the river bed in a saltation load. Instead, they remain in constant contact with the river bed and are transported through the force of the flowing water, gradually rolling or sliding along the river bed. This type of load requires a significant amount of force, as the rocks are heavy and require continuous contact with the river bed for transportation. Therefore, the heaviest rocks rolled along the river bed in river transport are referred to as traction load.