You use Azure Pipelines to build and deploy an application named App1 and created an Application Insights instance AI1. Which file in App1 should you modify to configure the application to use AI1?

In .NET Core applications, startup.cs is the correct file to modify because it contains the ConfigureServices method. This method is where you register services with the dependency injection container, including Application Insights. You would add code here to configure the Application Insights telemetry client with the instrumentation key or connection string from AI1. appsettings.json is used for application configuration settings, but the actual integration logic for Application Insights is typically handled in code. launchSettings.json defines project launch profiles for development environments and does not configure application services. project.json was an older project file format for .NET Core and has been replaced by .csproj files; it's not used for configuring Application Insights integration.