You need to meet the technical requirements for User1.

n the Administering Windows Server Hybrid Core Infrastructure guidance for managing AD DS, Microsoft emphasizes using OU-level delegation to satisfy administrative needs while adhering to the principle of least privilege. The documentation explains that the Delegate Control wizard on an OU lets you grant a user or group only the specific permissions required for common tasks, including “Modify the membership of a group”. This grants the write permission to the member attribute on group objects contained in that OU, without giving broader account-management rights across the domain.

By contrast, placing a user in Account Operators or Server Operators provides elevated, domain-wide capabilities far beyond what is required. Account Operators can create, delete, and modify many account types across the domain (except for protected admin accounts), which violates least-privilege for a task that only needs to change group membership in one OU. Server Operators is unrelated to group membership and is intended for server administration tasks. Creating a delegation at the domain root would similarly be excessive because it applies broadly to all containers and OUs.

Therefore, to meet the requirement “Ensure that User1 can manage the membership of all the groups in Contoso\OU3,” you should delegate control on OU3 and assign the built-in task “Modify the membership of a group” to User1, achieving the minimal permissions necessary.