The correct answer is: A. Avoid spoilage.
Phosphorus is a highly reactive element that can easily catch fire and explode. It is also poisonous if ingested. To prevent these hazards, phosphorus is typically stored in water. The water helps to keep the phosphorus cool and prevents it from coming into contact with oxygen, which can cause it to ignite. Additionally, the water helps to dissolve any phosphorus that may be released, making it less likely to be ingested by people or animals.
Option B is incorrect because phosphorus is not flammable when it is stored in water. Option C is incorrect because phosphorus is not a durable material. Option D is incorrect because phosphorus is not typically kept out of reach of children.