Final answer:
A jack stand locks when a steel pin engages with a toothed section, ensuring safety and stability. When the steel handle is released and the toothed section is lowered, it allows the steel pin to engage with one of the teeth, preventing any further movement.
Step-by-step explanation:
A jack stand is a device used to support a heavy load, such as a vehicle, after it has been lifted by a hydraulic jack or other means. The mechanism that locks a jack stand typically involves a toothed section, often referred to as the ratchet or the latching mechanism. This toothed section is locked in place when a steel pin, or pawl, engages with one of the teeth. Therefore, the correct answer is B. When a steel pin is positioned under one of the teeth, on the top of the jack stand, the jack stand is effectively locked in place, preventing it from collapsing and providing a safe support for the load.