The correct answer is option 'D', Opera.

Explanation:
An operating system (OS) is a software that manages computer hardware and software resources and provides common services for computer programs. It acts as an intermediary between the computer hardware and the user, allowing the user to interact with the computer system.

Let's examine each option to determine whether it is an operating system or not:

a) Android: Android is an operating system developed by Google primarily for mobile devices such as smartphones and tablets. It is based on the Linux kernel and is widely used by various device manufacturers.

b) Vista: Vista, or Windows Vista, is an operating system developed by Microsoft. It was released in 2007 as the successor to Windows XP. Although it has been replaced by newer versions of Windows, Vista was once a popular operating system.

c) iOS: iOS is an operating system developed by Apple Inc. It is used exclusively on Apple devices such as iPhones, iPads, and iPod Touch. iOS is known for its intuitive user interface and seamless integration with other Apple products and services.

d) Opera: Opera is not an operating system but rather a web browser developed by Opera Software. It is available for various operating systems, including Windows, macOS, and Linux. While it allows users to access the internet and browse websites, it is not an operating system itself.

In summary, among the given options, Opera is not an operating system. It is a web browser. The other options (Android, Vista, and iOS) are all operating systems used on different devices.