What adjustment must a system operator make to synchronize separated systems?

The correct choice involves adjusting area generation output to synchronize separated systems. When systems become unsynchronized, it often means that they are operating at different frequencies or voltage levels. By adjusting the generation output in one area, a system operator can effectively match the output of the disconnected system to ensure that both systems can synchronize efficiently.

This adjustment can help achieve balance in terms of load and generation, which is critical for maintaining system stability and preventing damage during the synchronization process. The operator may increase or decrease the generation output based on the disparity in frequencies or voltages to achieve a cohesive connection.

Other options provided may not be the ideal solutions in this context. For example, adjusting transformer tap changers could influence voltage levels but may not directly address synchronization issues across separated systems. Similarly, changing the power angle to 90 degrees or opening additional lines could lead to other complications and might not effectively ensure the required synchronization is achieved. Thus, focusing on the area generation output is the most direct and effective approach for this scenario.