______ is sometimes said to be object oriented, because the only way to…

Concept: An operating system is called object oriented in its design when every system resource is modelled as a typed object whose internal state is kept inside the kernel, and user code can reach that object only through an opaque reference together with the fixed set of methods the kernel publishes for that object type. That opaque reference is called a handle: it is an index into a per-process table that also records the access rights granted, so the kernel, and not the caller, enforces the type check and the permission check on every operation.

Application: The Windows NT line, the design Microsoft first shipped in 1993, is built exactly this way. Its executive is organised around a single Object Manager: the component that creates, names, secures and reference-counts the executive objects that user mode is allowed to hold, such as processes, threads, files, events, mutexes, semaphores, timers, registry keys and ports. Structures used only inside the kernel layer are not registered with the Object Manager and are never handed out at all. A program never receives the address of an executive object; calls such as CreateFile, CreateEvent and CreateMutex return a handle, and every later operation, such as WaitForSingleObject, ReadFile or CloseHandle, is a method invoked on that handle. Because the handle is the only door to the object, encapsulation is enforced by the operating system itself, which is what the sentence in the question describes.

Cross-check: The claim is deliberately hedged as “sometimes said to be”, and setting it beside the other releases shows why.

• The object model gives encapsulation and one uniform typed-method interface, but it has no inheritance and no polymorphism, so it is object-based rather than fully object oriented; that is why the statement is qualified.

• Windows 95 and Windows 98 run on an MS-DOS base and have no single kernel object manager; their resources are tracked by separate 16-bit and 32-bit subsystems instead.

• Windows XP (internally NT 5.1) and Windows Vista (internally NT 6.0) are later releases of that same NT line, so they inherit the object-and-handle model instead of introducing it. Because the option list offers the line itself alongside two of its own members, the line is what the sentence is naming.

Answer: Windows NT.