In a cloud environment, what controls inbound and outbound traffic between networks, particularly between virtual private clouds (VPCs)?

The correct choice addresses a fundamental aspect of network management and security in cloud environments, where controlling traffic between virtual private clouds (VPCs) is essential for maintaining proper security boundaries and optimizing resource usage.

Network Access Control (NAC) lists, often referred to in various contexts, are crucial for specifying which traffic is allowed or denied between different network segments. These lists can help enforce policies that dictate how data flows between VPCs, ensuring that only legitimate communication occurs based on defined security protocols.

In the context of cloud architecture, these access controls are vital for preventing unauthorized access and protecting data integrity. They function by filtering requests based on various attributes like IP addresses, protocols, and ports, thus creating a layered security approach that aligns with best practices in cybersecurity.

In contrast, the other options relate to different aspects of cloud and network security. For instance, Cross Domain Solutions are typically used to enable secure data flows between networks with differing security levels but do not specifically address the granular control of traffic between VPCs. Directory services facilitate user and resource management but are not focused on traffic regulation. Identity Providers assist with authentication and access controls for users, but again, they do not manage network traffic between VPCs directly.