Final answer:
In French, the pronoun 'en' replaces a prepositional phrase that starts with 'de'.
Step-by-step explanation:
In French, the pronoun 'en' is used to replace a prepositional phrase that starts with the word 'de' and indicates an amount or quantity. In the given question, 'Louis, achètes-tu une bouteille d'eau minérale?', the prepositional phrase 'd'eau minérale' can be replaced by 'en'.
So the correct answer would be 'Oui, j'en achète quatre pour notre pique-nique.' Here, 'j'en' replaces 'd'eau minérale' and it indicates that Louis is buying four bottles of mineral water.
Learn more about French pronouns