Reference:
https://github.com/MicrosoftDocs/azure-docs/blob/master/articles/app-service/web-sites-scale.md Scale up : Correct Choice Scale up: Get more CPU, memory, disk space, and extra features like dedicated virtual machines (VMs), custom domains and certificates, staging slots, autoscaling, and more. You scale up by changing the pricing tier of the App Service plan that your app belongs to.
Scale out : Incorrect Choice
Scale out: Increase the number of VM instances that run your app. You can scale out to as many as 30 instances, depending on your pricing tier. App Service Environments in Isolated tier further increases your scale-out count to 100 instances. For more information about scaling out, see Scale instance count manually or automatically.
Add continuous WebJobs : Incorrect Choice
WebJobs is a feature of Azure App Service that enables you to run a program or script in the same instance as a web app, API app, or mobile app. Add continuous WebJobs will Starts immediately when the WebJob is created. To keep the job from ending, the program or script typically does its work inside an endless loop. If the job does end, you can restart it.Starts only when triggered manually or on a schedule.
Add a virtual machine scale set : Incorrect Choice
A virtual machine scale set allows you to deploy and manage a set of identical, autoscaling virtual machines. You can scale the number of VMs in the scale set manually. You can also define rules to autoscale based on resource usage such as CPU, memory demand, or network traffic. It will not increase the slowness of the apps.
https://docs.microsoft.com/en-us/azure/app-service/manage-scale-up
https://docs.microsoft.com/en-us/azure/app-service/webjobs-create#webjob-types