Your company wants to deploy 2-step verification (2SV).

Comprehensive and Detailed Explanation From Exact Extract:

The goal is to deploy 2SV with a phased rollout to control the number of individuals affected at once, minimizing disruption, and keeping management simple.

While OU structure can be used, managing policy exceptions based on Configuration Groups is the recommended and less complex way to handle phased rollouts of security settings like 2SV in Google's Admin console.

Extracts:

"When rolling out 2SV, it is highly recommended to use a phased deployment approach to manage the impact on user access and support resources." (Source 2.1)

"You can use configuration groups to select a subset of users within an OU structure and apply specific settings, such as 2SV enforcement, to them. This allows for a gradual, controlled rollout without needing to alter your primary organizational unit structure." (Source 2.2)

Groups allow you to easily add/remove users from the enforcement scope without moving their identity within the OU hierarchy (Option A), which keeps management complexity low. Options C and D do not allow for the necessary phased control of enforcement.