Final answer:
The correct cmdlet to create the forest root domain on a Windows Server 2016 after installing AD DS is Install-ADDSForest. It initiates the setup of the forest and first domain controller.
Step-by-step explanation:
To create the forest root domain on a standalone Windows Server 2016, after the Active Directory Domain Services (AD DS) has been installed, the correct PowerShell cmdlet to use is Install-ADDSForest. This cmdlet is part of the ADDSDeployment module and is utilized for the initial creation of the AD DS forest, this process includes the setup of the first domain controller in the forest. Using Install-ADDSForest, parameters like domain name, safe mode administrator password, and database paths can be specified to customize the installation.