The correct answer is D) Immobilize a fracture. The primary purpose of applying a splint in first aid is to immobilize a suspected fracture. By immobilizing the injured limb, the splint helps prevent further damage to the bone, muscle, and surrounding tissues. This is crucial for stabilizing the injury and reducing the risk of complications during transport to medical care. The other options are incorrect because: A) Resetting the bone requires medical intervention and should not be attempted by a first aid provider; B) Reducing pain is a secondary benefit of splinting, not the primary purpose; and C) Controlling bleeding is addressed through other first aid techniques, not the application of a splint.