What device is primarily used by the driver to control the speed of the vehicle?

The accelerator pedal is the primary device that a driver uses to control the speed of the vehicle. When the driver presses down on the accelerator, it increases the engine's power output, allowing the car to accelerate and gain speed. This direct correlation between the accelerator and the vehicle's speed is fundamental to the operation of any vehicle, as it enables the driver to adjust speed in response to traffic conditions, road requirements, and driving intentions.

In contrast, while the brake pedal is essential for slowing down or stopping the vehicle, it does not control speed in terms of acceleration; instead, it acts as a decelerator. The clutch, commonly found in manual transmission vehicles, is mainly used to engage and disengage the engine from the wheels, allowing for gear changes rather than direct speed control. The hood-release lever has no direct relevance to vehicle speed; it is simply a mechanism used to open the hood for maintenance or inspection. Thus, the accelerator pedal is distinctly the correct answer for controlling the speed of a vehicle.