A distributed operating system typically consists of:

Explanation

The hardware of a distributed system is loosely coupled: independent machines/processors, each with its own private memory, connected only by a network, sharing no common memory or clock. Despite that, the operating-system software layer running across those machines is tightly coupled — it cooperates closely to present a single, unified system image and manage resources transparently across the network. So a distributed OS pairs tightly coupled software with loosely coupled hardware, option (iii); pairing the tightness of the software with matching hardware tightness instead describes ordinary tightly/loosely coupled multiprocessor systems, not a distributed system.