The user with an iOS issue most likely has an iPhone since iOS is the operating system for Apple's mobile devices. Other options mentioned like iCloud, Android Phone, or Windows Tablet don't run iOS.
If a user is concerned about an issue with iOS, the user MOST likely has a C. iPhone. iOS is the operating system that runs on Apple's mobile devices, specifically the iPhone. As a software platform, iOS is not used on iCloud, Android Phones, or Windows Tablets, which either use different operating systems like Android and Windows or are services rather than devices in the case of iCloud.