What tool should Company A and Company B use to share Azure resources after their merger?

The most suitable tool for Company A and Company B to share Azure resources after their merger is Azure Resource Manager. Azure Resource Manager is designed to manage and organize resources in Azure and allows organizations to create, update, and delete resources in a consistent manner. This tool enables resource sharing through resource groups, which allows multiple subscriptions and resources to be managed efficiently in a unified way.

Azure AD Connect Cloud Sync is primarily used for synchronizing on-premises Active Directory objects to Azure Active Directory and might not directly facilitate resource sharing between two companies. Azure Traffic Manager is a traffic load balancer that helps distribute network traffic to different services but does not provide a means for sharing Azure resources directly. Azure Active Directory B2C is focused on managing consumer identities and access to applications, rather than resource management and sharing between organizations.

By utilizing Azure Resource Manager, both companies can effectively manage and share their resources post-merger, allowing for streamlined operations and consolidated resource management.