The correct option is option 2, kerosene. Alkali metals are highly reactive and can react vigorously with air and water, so they need to be stored in a substance that can protect them from these elements. Kerosene is commonly used for this purpose because it acts as a barrier to prevent the alkali metals from coming into contact with air and moisture. It forms a layer on the surface of the alkali metals, effectively preventing them from reacting. This allows the alkali metals to be safely stored and handled.

Option 1, petrol, is not an appropriate choice because it is highly flammable and can react with alkali metals, posing a safety hazard. Option 3, water, is also not a suitable option because alkali metals react violently with water, producing hydrogen gas and heat. Option 4, absolute alcohol, is not commonly used for storing alkali metals because it is not as effective as kerosene in preventing their reaction with air and moisture. Therefore, kerosene is the most appropriate option for storing alkali metals safely.