A valid reason for moving to Azure Active Directory Domain Services (AAD DS) would be:
B. You are currently experiencing a lack of available local infrastructure.
Moving to AAD DS can provide a solution when there is a limited availability of local infrastructure, such as on-premises domain controllers. By utilizing AAD DS, you can leverage the cloud-based domain services provided by Azure, eliminating the need to maintain and manage on-premises infrastructure.
While AAD DS offers some security benefits, it's not the primary reason for moving to it. The cost comparison between on-premises and Azure depends on various factors, so it may not always be accurate to say it will cost less.
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