The correct answer is: Water.
White phosphorus is a highly flammable and toxic substance. It is stored in water to prevent it from self-igniting. When white phosphorus comes into contact with air, it reacts with oxygen to form phosphorus pentoxide, which is a white powder that is highly flammable. The presence of water prevents the formation of phosphorus pentoxide by absorbing the oxygen in the air.
Option A: Oxygen is a gas that is essential for combustion. If white phosphorus were stored in oxygen, it would be more likely to self-ignite.
Option B: Hydrogen is a gas that is also highly flammable. If white phosphorus were stored in hydrogen, it would be even more likely to self-ignite.
Option C: Water is a liquid that is non-flammable and does not react with white phosphorus. It is therefore the safest substance to store white phosphorus in.
Option D: Alcohol is a flammable liquid. If white phosphorus were stored in alcohol, it would be more likely to self-ignite.