Start additional VM instances to host the application is correct when you scale out a web app.

Scaling out increases the number of running instances that handle traffic so the application can serve more requests and improve availability. The platform provisions additional virtual machines or workers and spreads load across them to increase capacity and fault tolerance.

Deploy the app to another region for global routing is incorrect because scaling out does not automatically move or replicate apps across geographic regions. Multi region deployments and global routing require separate configuration and are not the same as adding more instances.

Upgrade the App Service Plan to a higher tier such as moving from S1 to S2 is incorrect because that action is called scaling up and it increases the size or power of the existing machines rather than the number of instances.

Create extra app replicas within the same VM instance is incorrect because scale out creates additional VM instances or workers. Creating more replicas inside a single VM does not provide the same isolation or capacity benefits as adding full instances.

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