You need to start the planning process by exploring Azure. What should (AZ-900)

Correct answer: A) A subscription.
Before you can start creating resources in Azure, you need to have an Azure subscription. An Azure subscription is a logical grouping of Azure accounts and resources that are associated with a single billing account. It provides access to Azure services and allows you to create and manage resources.
Once you have a subscription, you can create resource groups, virtual networks, management groups, and other resources. A resource group is a logical container that holds related resources, a virtual network is a virtual representation of a traditional network, and a management group is a container that holds subscriptions.

So, the correct order is:
Create an Azure subscription (A)
Create a resource group (B)
Create a virtual network (C)
Create a management group (D)

Note that you may not need to create a management group immediately, as it's used to group subscriptions for governance and billing purposes. But you do need a subscription to start creating resources in Azure.