When upgrading an existing forest, what is the correct order for upgrading domain controllers?

Upgrading domain controllers in the correct order is crucial for maintaining the integrity and functionality of an Active Directory environment. The recommended approach is to upgrade the domain controllers in the forest root domain first. This is because the forest root domain contains critical information that affects the entire forest, including the schema and configuration partitions.

By upgrading the forest root domain controllers first, you ensure that the new functionalities and capabilities of the upgraded version can be fully leveraged throughout the other domains. The forest root acts as the anchor for the entire directory service, and if its controllers are outdated, it can lead to compatibility issues and hinder the functionality of child domains during the upgrade process.

Once the forest root is successfully upgraded, you can proceed with upgrading the child domain controllers. This sequential approach minimizes the risk of issues arising and helps ensure a smooth transition to the new environment.

Other options, while they may seem plausible, do not follow the best practices for upgrading Active Directory environments, leading to potential risks such as service disruptions or feature compatibility problems.