Engine synchronization is enabled at:

When considering engine synchronization in the CRJ550, it's essential to understand the operational parameters that dictate when synchronization can be activated. Engine synchronization is a system designed to ensure that the thrust levels of both engines are matched as closely as possible, promoting smoother operation and fuel efficiency.

The synchronization is typically activated at specific RPM thresholds for the engines, utilizing parameters such as N1 and N2. In this case, the correct response highlights that synchronization can be enabled at two different conditions: N1 at 1.5% when the left engine is designated as the master, and N2 at 2.5% also reflecting the left engine as master. This dual capability allows pilots flexibility in managing engine performance, ensuring that they can optimize thrust settings whether they are operating in a takeoff or cruise phase.

While the other options may suggest different values or configurations for synchronization, the correct choice encompasses the broader operational context in which the left engine can be the master at two different N1 and N2 settings, allowing for versatility in managing power between the engines.