Which option provides the best way to ensure redundancy for a web app on Azure?

Utilizing multiple instances in an Availability Set is the best way to ensure redundancy for a web app on Azure. This approach allows for multiple virtual machine instances to be deployed in such a way that they are spread across different physical hardware within the same Azure data center. By doing this, if a single hardware failure occurs, the other instances can remain operational, thereby providing high availability for the application. The Availability Set also ensures that the instances are not taken down for maintenance at the same time, further enhancing reliability.

In contrast, hosting the web app on a single virtual machine creates a single point of failure, meaning that if that VM goes down, the entire application becomes unavailable. Using Azure Functions with scaling can provide some level of redundancy and can respond to load changes, but it is more about scaling rather than focused redundancy for web app availability. Deploying the app across multiple Azure regions does enhance the geographical redundancy and can protect against region-wide outages, but it is generally more complex and costly for simple web app scenarios compared to using an Availability Set within a single region. Thus, for effective redundancy with a straightforward approach, deploying multiple instances in an Availability Set is the most effective strategy.