Water is a good coolant for engines because it has a high specific heat. Specific heat refers to the amount of heat energy that a substance can absorb or release without a significant change in temperature. In the case of water, it has a high specific heat, which means it can absorb a large amount of heat energy before its temperature increases significantly.

When water is used as engine coolant, it absorbs the excess heat produced by the engine and carries it away, preventing overheating. This helps in maintaining the engine temperature within a safe range and prevents damage to the engine components.

Option 2, low surface tension, is not the reason why water is a good coolant for engines. Surface tension is a property that affects how water molecules behave at the surface of a liquid, and it is not directly related to its cooling abilities.

Option 3, high boiling point, and option 4, low expansivity, are also not the reasons why water is a good coolant. While high boiling point and low expansivity may have some impact on water`s effectiveness as a coolant, they are not the primary factors. The high specific heat of water is the main reason why it is an excellent coolant for engines.

Therefore, the correct answer is option 1.