Which of the following controls the way in which the computer system functions and provides a means by which users can interact with the computer.

Every computer needs a piece of software that manages its hardware resources and gives the user a way to interact with the machine, since without it neither the hardware nor any application program could function usefully.

The operating system is exactly this piece of system software. It controls how the computer's hardware resources, such as memory, storage, and processing time, are allocated and used, and it provides the interface (whether graphical or command-based) through which a user gives commands and sees results. Familiar examples include Windows, macOS, Linux, and Android.

The other options do not fit this description. The CPU and the ALU are hardware components that carry out instructions and calculations; they do not manage overall system function or provide a user interface on their own. Application software (such as a word processor or spreadsheet) lets a user perform specific tasks, but it runs on top of, and depends on, the operating system rather than controlling the way the whole computer system functions.

When a question links "controlling how the system functions" with "providing a means for users to interact" in the same sentence, that combination of roles is the defining job of the operating system.