What type of trust relationship should be created for three subsidiaries having their own forests?

In scenarios involving three subsidiaries that each have their own forests, the most appropriate type of trust relationship to establish is a forest trust. A forest trust allows for resources and identities to be shared across the entire forest, which is particularly beneficial for interconnected organizational structures such as subsidiaries.

Forest trusts facilitate trust relationships between two separate Active Directory forests, enabling users in one forest to access resources in another forest without the need for duplicate identities. This type of trust is efficient for larger organizational hierarchies, particularly when the subsidiaries need access to resources across each other's forests.

The chosen answer, external trust, typically applies to scenarios where trusts need to be created with external organizations or non-Active Directory domains, making it less ideal for the situation with internal subsidiaries structured within separate forests. Thus, for interconnecting subsidiaries that operate independently within their own forests, a forest trust would be essential for optimal resource sharing and user authentication capabilities.