💡 Detailed Explanation
The correct choice is '理由' (riyū - reason). Sentence translation: 'The reason why he couldn't come to the party is that he was busy with work.' The sentence ends with '〜からです' (because...), which pairs naturally with '理由' (The reason is because...). '原因' (gen'in - cause) is used for physical or negative causal relationships (e.g., the cause of an accident). '結果' (kekka) means result. '関係' (kankei) means relation/connection.