What type of authentication must be selected to allow all users in the trusted domain access to resources?

In a scenario where you want to allow all users in a trusted domain access to resources, forest-wide authentication is the appropriate choice. This type of authentication allows any user from a trusted domain to access resources across the entire forest without additional restrictions.

Forests are the top level of Active Directory structure, and granting forest-wide authentication means that all users, regardless of which domain in the forest they belong to, will have the ability to authenticate and access resources seamlessly. This is particularly useful in environments where collaboration and resource sharing across multiple domains is necessary.

The other options provide varying levels of authentication granularity. Domain-wide authentication limits access to users within the specific domain and does not extend to other domains. Selective authentication requires explicit permissions to be granted for each user or group in a trusted domain, making it more complex if many users need access. Object-level authentication is not a standard term in Active Directory security contexts, typically referring to specific security settings on individual objects rather than broad access permissions like those managed in domains or forests. Thus, for broad and unrestricted access for all users in trusted domains, forest-wide authentication is the optimal choice.