An Azure Resource Manager (ARM) template is an Infrastructure as Code (IaC) file used to declaratively define and deploy Azure resources. To automate the deployment of 100 virtual machines and ensure they are joined to an on-premises Active Directory Domain Services (AD DS) domain, an ARM template is the most suitable tool. The template can include the JsonADDomainExtension virtual machine extension. This extension runs a script during the VM provisioning process that automatically joins the machine to the specified AD DS domain, providing a scalable and consistent deployment method.

A. Azure AD Connect: This tool synchronizes identity objects between an on-premises AD DS and Azure AD. It does not deploy or configure Azure virtual machines.
B. a Group Policy Object (GPO): GPOs apply configuration settings to computers that are already members of an AD DS domain. They cannot be used to perform the initial domain join action.
D. an Azure management group: Management groups are a governance tool for organizing Azure subscriptions and applying policies or access controls at scale. They do not directly deploy or configure resources.

1. Microsoft Docs

"JsonADDomainExtension for Windows": This document describes the VM extension used to join an Azure VM to an Active Directory domain. The "Template deployment" section explicitly states

"Azure VM extensions can be deployed with Azure Resource Manager templates."

Reference: Microsoft Corporation. (n.d.). JsonADDomainExtension for Windows. Microsoft Learn. Retrieved from https://learn.microsoft.com/en-us/azure/virtual-machines/extensions/json-ad-domain-extension

2. Microsoft Docs

"Tutorial: Create an Azure VM that is joined to a domain": This official tutorial provides a step-by-step guide that uses an ARM template and the JsonADDomainExtension to automate the creation of a Windows Server VM and join it to a domain.

Reference: Microsoft Corporation. (n.d.). Tutorial: Create an Azure VM that is joined to a domain. Microsoft Learn. See the "Deploy the template" section. Retrieved from https://learn.microsoft.com/en-us/azure/architecture/reference-architectures/identity/adds-extend-domain#deploy-the-template

3. Microsoft Docs

"What is Azure AD Connect?": This document defines the purpose of Azure AD Connect

clarifying that it is for synchronizing on-premises directories with Azure AD and is not involved in resource deployment.

Reference: Microsoft Corporation. (n.d.). What is Azure AD Connect?. Microsoft Learn. See the "What is Azure AD Connect?" section. Retrieved from https://learn.microsoft.com/en-us/azure/active-directory/hybrid/connect/whatis-azure-ad-connect