The correct answer is 'Assessing the effectiveness of the organization's threat hunting practices'. In the case of an APT, where the adversary has managed to stay undetected for an extended period, it is imperative to evaluate the threat hunting capabilities, as these are designed to proactively detect and isolate sophisticated threats that evade traditional security measures. Reviewing patch management policies might be an appropriate measure, but it is less specific to the given scenario of an APT which typically circumvents such defenses. Evaluating the encryption methods for data at rest is a security best practice but does not directly address the issue of detection and prolonged unauthorized access. Implementing a network segmentation strategy could help to contain movement within the network, but it would not necessarily identify the root cause of how the APT remained undetected.