What is iOS used for?

iOS is specifically designed as the operating system that powers Apple's mobile devices, including iPhones and iPads. Its architecture is optimized for touch interfaces and mobile applications, making it suitable for a wide range of tasks that users perform on their devices, such as accessing apps, making phone calls, browsing the web, and using various integrated services. The operating system manages hardware resources and provides a framework for developers to build mobile applications.

In contrast, running macOS pertains to computers like MacBook and desktop Macs, and managing cloud services generally involves software and applications that handle data storage and retrieval across the internet. Facilitating Apple music streaming is a function of specific applications and services like Apple Music rather than the operating system itself. Thus, the primary role of iOS is to operate Apple mobile devices, making option C the correct choice.