The correct answer is D. Water and petrol are immiscible with each other and petrol which forms the upper layer continues to burn.
Water is not effective in extinguishing a fire caused by petrol because water is immiscible with petrol. This means that water and petrol do not mix together. When water is poured on a petrol fire, the water will float on top of the petrol and will not be able to reach the flames. The petrol will continue to burn and the fire will spread.
The other options are incorrect. Option A is incorrect because water can cool down a flame. However, water cannot cool down a petrol fire because the water will not be able to reach the flames. Option B is incorrect because water and petrol do not react chemically. Option C is incorrect because water and petrol are immiscible with each other.