What type of operating system is commonly found on Apple iPhones?

The type of operating system commonly found on Apple iPhones is a closed-source Unix-based operating system. This classification is accurate because iOS, which is the operating system used by Apple iPhones, is built on a Unix-like foundation known as Darwin. Unix-based systems are known for their stability, security, and multi-user capabilities, which are essential features for mobile devices.

Moreover, the closed-source nature of iOS means that the source code is not publicly available or modifiable by users, allowing Apple to maintain control over the software and ensure consistency across its devices. This approach protects the system from vulnerabilities that might arise from open-source projects while also allowing Apple to implement proprietary features and a seamless user experience within its ecosystem.

In contrast, other options fall short in accurately describing iOS. Open-source OS implies that the software’s source code is available for modification and redistribution, which is not the case with iOS. Free and widely available suggests a model where the software can be freely distributed and used, which does not align with Apple's business model as iOS is exclusive to Apple devices. Lastly, a web-based OS refers to an operating system that primarily functions through internet connectivity and browser-based applications, which is not representative of iOS's capabilities or structure