Final answer:
The interface for accessing the services of an operating system is provided by system calls.
Step-by-step explanation:
The correct answer is system calls. To access the services of an operating system, an interface is provided by system calls. System calls are functions provided by the operating system that can be called by user programs to request services from the operating system.
For example, if a program needs to read data from a file, it can make a system call to the operating system to perform the file read operation.
A system call provides a low-level interface between the user program and the operating system, allowing the program to interact with the underlying resources and functionality of the operating system.