In which situation is it appropriate to use water to extinguish a fire?

Using water to extinguish fires in a waste paper trash can is appropriate because these types of fires typically involve combustible materials like paper, cardboard, or lightweight plastics that do not react dangerously to water. Water is effective in cooling and smothering these materials, which helps to eliminate the heat source and extinguish the flames.

In contrast, using water on electrical fires can create a significant risk of electrocution, as water conducts electricity. Kitchen grease fires require specific agents that can smother the flames, as water can cause the burning grease to splatter, spreading the fire. Vehicle fires often contain flammable fluids and components that can react negatively with water, making it unsafe to use in those situations. Therefore, utilizing water for a fire in a waste paper trash can is the only safe and effective option among the listed scenarios.