Step 1: Recall phosphorus's biggest danger.
White phosphorus catches fire on its own in air at temperatures as low as about $35^\circ C$, close to a hot day's room temperature, so keeping it exposed to air is not an option.
Step 2: Rule out air and alcohol.
Open air obviously supplies the oxygen it needs to ignite, so that is unsafe. Alcohol is flammable itself and would only add fuel near a substance this reactive.
Step 3: Check water as the storage medium.
Phosphorus does not react with water and does not dissolve in it, so submerging it under water cuts it off from atmospheric oxygen completely without triggering any reaction. This is different from sodium or potassium, which are stored in kerosene precisely because they do react violently with water.
Step 4: Conclude.
Phosphorus is stored under water. \[ \boxed{\text{Water}} \]