You need to configure the web app contoso.azurewebsites.net to host the custom domain www.contoso.com. What should you do first?

Before you can map a custom domain to an Azure web app, you must verify ownership of the domain. This is done by creating a TXT record in your domain's DNS zone. Azure provides a unique domain verification ID, which you use as the value for a TXT record named asuid. This record proves to Azure that you control the domain. Once verified, you can then proceed to create the necessary CNAME or A records to point the custom domain to your web app. Creating A records directly (option A) or a CNAME record named asuid (option C) are not the initial steps for domain verification. Creating a TXT record named www.contoso.com with the web app's URL (option D) is incorrect for verification and would typically be a CNAME record for the actual mapping.