Final answer:
The over-arm head splint is the correct in-line stabilization technique for a responsive victim with a suspected head, neck, or spinal injury when using a backboard.
Step-by-step explanation:
When placing a responsive victim with a suspected head, neck or spinal injury on a backboard, the correct in-line stabilization technique to use is c. The over-arm head splint. This technique involves one rescuer at the victim's head maintaining manual stabilization, while the victim's head and torso are secured to the board as a single unit. The over-arm head splint is a method where the rescuer's arms are positioned over the victim's shoulders, with the hands cradling the sides of the head, to keep the head, neck, and spine in a straight line during the placement and securing process. This avoids any twisting or bending movements that could potentially cause further injury.