Which of the following measures involves preventing unauthorized access to a network?

Network Access Control (NAC) is designed specifically to prevent unauthorized access to a network by enforcing security policies at the network level. It achieves this by ensuring that only devices that comply with specific security requirements can connect to the network. This includes checking for up-to-date antivirus software, security patches, and other compliance measures. Additionally, NAC often includes mechanisms for segmenting the network, isolating non-compliant devices, and providing access to only certain parts of the network based on each device's security posture.

In contrast, while intrusion detection systems can alert administrators to unauthorized access attempts, they do not actively prevent access. Data encryption secures data by making it unreadable to unauthorized users, but it does not prevent access to the network itself. Application security focuses on protecting applications from threats but does not directly address unauthorized network access. Therefore, Network Access Control stands out as the most effective measure for directly preventing unauthorized access to a network.