The correct answer is C) flow the foam down a nearby vertical surface. The reason this is the correct method is that it allows the foam to spread across the surface of the fire and smother it effectively. Flowing the foam down a nearby vertical surface creates a blanket of foam that then spreads out over the burning material, depriving the fire of oxygen and extinguishing it. This is the recommended technique as per Coast Guard regulations and fire safety best practices. The other options are less effective - spraying directly on the surface or base of the fire can disrupt the fire and cause it to spread, while sweeping the fire with foam may not provide sufficient coverage to fully extinguish the flames.