Step 1: Understand the mode boundary.
An operating system runs its core code in a protected kernel mode, while ordinary applications run in a restricted user mode that cannot directly touch hardware or sensitive OS structures. This separation keeps the system safe and stable.
Step 2: Ask how a user program crosses that boundary.
Whenever a user program needs an OS service, like reading a file, creating a process, or sending network data, it cannot do this on its own. It must make a formal request that temporarily switches the CPU into kernel mode so the OS can safely perform the task and then return control to the program. This formal request mechanism is the system call interface.
Step 3: Rule out the other components.
Device drivers are internal kernel modules that talk to hardware, they are not the interface applications use directly. Libraries are collections of reusable functions, and the Process Control Block simply stores a process's bookkeeping data, neither of these is the mechanism by which user programs request OS services.
Step 4: Conclude.
The interface that lets user processes access OS services is
\[ \boxed{\text{System Calls}} \]