Hast du Geschwister, Laura? Ja, ich habe ... Bruder. Er wohnt aber nicht mehr bei uns. die eine ein einen der

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The question is asking in German if Laura has siblings. Following the scenario indicated, the blank space should be filled by "einen" (auf Englisch "a"). We are talking about a direct object (the brother), and in addition, the German requires that nouns use a definite or indefinite article. The word 'Bruder' is masculine and in the accusative case—which in German is the case used for direct objects—the indefinite article 'ein' changes to 'einen'. Thus, 'einen' is the correct usage.