Which model describes a scenario where the database and application exist on a single system?

The one-tier model is characterized by the database and application residing on the same system, meaning that both the application interface and its data source are contained within a single layer. This setup typically simplifies the architecture, as everything necessary for the application to function is local, allowing for direct access to the database without intermediate layers or services.

In this environment, the application can directly read from and write to the database, providing efficient communication with reduced latency due to the absence of network delays that occur in multi-tier models. This model is often suitable for small applications or development environments where simplicity and speed are prioritized, making it effective for certain use cases.

On the other hand, the two-tier and three-tier models introduce additional layers, with distribution of components across different systems, which can increase complexity and provide scalability advantages in larger applications. The cloud model refers specifically to a service delivery model over the internet, which does not fit the description of local database and application interaction. Thus, the one-tier model accurately describes the scenario stated in the question.