To troubleshoot a Compute Engine image that isn't booting, the most effective first step is to check the serial port logs. Google Cloud provides serial console output, which captures boot-time messages, kernel errors, or service failures. Option D suggests checking Compute Engine OS logs via the serial port and validating access through Cloud Logging. This approach is non-destructive and directly addresses boot-related issues. Other options, like rebuilding the image locally (C) or exporting the disk (B), are less immediate or more complex. Option A (rebuilding in a new branch) skips root-cause analysis. Thus, D is the best choice for initial investigation.