Four primary database models exist: - Relational (A): Data is structured into tables (relations) with interconnections established via foreign keys. - Hierarchical (B): Data is arranged in a tree structure, defining parent-child relationships for each record. - Network (C): Data is organized as a graph, with nodes representing entities and edges signifying their relationships. - Sequential (D) is not a database model. It describes a data access method where records are processed sequentially, commonly found in flat-file databases, but not a distinct model type. Consequently, the correct selection is (D).