Which of the following best defines an "instance" in cloud computing?

An "instance" in cloud computing specifically refers to a virtual machine that is provisioned and running within a cloud environment. This definition is pivotal because virtual machines are the core unit of computing resources in cloud services, representing a software-based emulation of a physical computer. Each instance includes an operating system, applications, and data, allowing users to run software in a flexible, scalable, and on-demand manner, without needing to manage the underlying hardware.

This understanding is crucial, as cloud computing often employs a pay-as-you-go model, where users can spin up or down instances based on their needs, making it a cost-effective solution for development, testing, and production environments. Instances can vary in size, capabilities, and configuration, enabling a high degree of customization to meet specific requirements.

In contrast, the other options do not accurately encapsulate the concept of an instance in cloud computing. A complete physical server refers to hardware and does not engage with the virtualization aspect. Any data application hosted online does not necessarily imply an instance, as it could encompass multiple components and services beyond just a virtual machine. A user account in a cloud service is more related to access and authentication rather than the infrastructure aspect that an instance represents.