A security engineer is setting up a security solution that can enforce mandatory access controls between two connected sites. Which of the following should the engineer implement?

The most appropriate implementation for enforcing mandatory access controls between two connected sites is a Cross Domain Solution (CDS). A CDS is specifically designed to facilitate secure information sharing across networks with differing security classifications. It allows for controlled access in environments where data sensitivity varies, ensuring that policies governing data access and flows are strictly adhered to.

In a scenario where two sites need to communicate while maintaining rigorous security measures, a CDS provides mechanisms to mediate and monitor this interaction, effectively enforcing the necessary access controls. It does this through a combination of security policies, data filtering, and data sanitization to ensure that only authorized information is transmitted between different domains.

Additionally, while directory services, identity providers, and NAC lists (Network Access Control Lists) play vital roles in overall network security and identity management, they do not specifically focus on the enforcement of mandatory access controls across distinct security domains. Directory services and identity providers facilitate authentication and user management, while NAC lists primarily control access to the network based on defined rules rather than inter-domain data control. Thus, a CDS is uniquely suited for the requirements outlined in the question.