The correct answer is "keluarga". The explanation for this is that the family plays a crucial role in the socialization process of a child. It is within the family that children learn about faith, values, and social norms. The family provides the foundation for a child's moral development and helps shape their personality. While other factors such as school, society, government, and mass media also contribute to socialization, the family has the most significant influence on a child's upbringing and character formation.