Which of the following is an API?

An Application Programming Interface (API) serves as an intermediary that allows different software programs to communicate with one another. The correct answer highlights that an API enables applications to perform needed tasks without granting them direct access to underlying hardware, which is crucial for maintaining security and stability within an operating system.

This abstraction layer ensures that applications can request services and data from the hardware through predefined functions without having to manage hardware resources directly. This protects the system from potential errors or malicious actions that could arise if applications had unrestricted control over hardware components. The use of APIs is fundamental in operating system design, as they encapsulate complex interactions into manageable, secure interfaces that developers can use to build sophisticated applications without delving into low-level programming.

The other choices describe functions or abilities related to programming and system interaction but do not accurately represent the primary purpose of an API. For instance, direct memory manipulation is not typical or safe for application design due to the risk of instability or security concerns, while direct hardware access would compromise the protective layers modern operating systems employ to safeguard against errant applications. An API ultimately serves as a controlled interface for interaction rather than direct access.