"In Active Directory, what is an organizational unit?
An organizational unit (OU) is a subdivision within an Active Directory into which you can place users, groups, computers, and other organizational units. You can create organizational units to mirror your organization's functional or business structure. Each domain can implement its own organizational unit hierarchy. If your organization contains several domains, you can create organizational unit structures in each domain that are independent of the structures in the other domains.
The term "organizational unit" is often shortened to "OU" in casual conversation. "Container" is also often applied in its place, even in Microsoft's own documentation. All terms are considered correct and interchangeable.
At Indiana University, most OUs are organized first around campuses, and then around departments; sub-OUs are then individual divisions within departments. For example, the BL container represents the Bloomington campus; the BL-UITS container is a subdivision that represents the University Information Technology Services (UITS) department on the Bloomington campus, and there are subcontainers below that. This method of organization is not an enforced rule at IU; it is merely chosen for convenience, and there are exceptions."
Fonte: http://kb.iu.edu/data/atvu.html
Fiz esse concurso... E adivinha!? Lasquei-me nessa questão infeliz, fui logo marcando Domínios.
A pegadinha da questão está na palavra "contêineres". Na verdade um contêiner é uma Unidade Organizacional!
Uma Unidade Organizacional (UO) é criada quando queremos fazer uma divisão lógica de um domínio, para que possamos delegar "poderes administrativos" diferentes no domínio. Por exemplo, podemos criar as UOs (UO Administrativo, UO Financeiro e UO Contabilidade) e assim restringir os direitos de acesso para cada tipo diferente de usuário em uma empresa, por exemplo.