Which of the following is NOT a layer of the OSI model?

The correct choice identifies "Database" as not being a layer of the OSI model. The OSI (Open Systems Interconnection) model is a conceptual framework used to understand and implement network communications. It defines seven distinct layers, each responsible for different aspects of data transmission.

The layers of the OSI model are:

1. Physical

2. Data Link

3. Network

4. Transport

5. Session

6. Presentation

7. Application

Each layer serves a specific function, and they work together to enable communication between networked systems.

"Database" does not fit within this framework, as it typically refers to an organized collection of data, often managed by database management systems (DBMS), rather than being part of the networking model itself. This distinction is important because understanding the roles of various layers helps clarify how networking protocols and services interact and operate.

The other options, "Application," "Presentation," and "Transport," all correspond directly to the official layers defined by the OSI model. The Application layer provides network services to end-user applications, the Presentation layer handles data translation and encryption, and the Transport layer manages end-to-end communication and data flow control.