What do system APIs specifically provide to developers?

System APIs provide developers with a set of functions to access operating system features. This is crucial because it allows applications to interact seamlessly with the underlying system, enabling functionalities such as file management, process control, network communication, and hardware manipulation. By utilizing system APIs, developers can create software that not only functions effectively but also integrates well with the operating system, leveraging its capabilities without needing to understand the complex underlying architecture.

The other options focus on different aspects of software development. Designing graphical interfaces relates to UI frameworks rather than core OS access. Data compression is a specific functionality that may be implemented through different libraries, but it is not a primary purpose of system APIs. A user manual for the OS is intended for user guidance rather than for providing programmatic access to functionalities, which is the primary role of system APIs. Therefore, the correct answer emphasizes the critical role system APIs play in application development concerning operating system interactions.