When a Compute Engine instance is unresponsive to SSH, the next step should be to diagnose the root cause. Enabling and checking the serial port output (Option B) allows access to low-level logs (e.g., kernel messages, boot errors, or OS crashes) even if SSH is unavailable. Rebooting (Option A) might resolve transient issues but risks losing critical diagnostic information. Deleting (C) or snapshotting (D) are recovery steps but do not address immediate diagnosis. Thus, Option B is the most appropriate next action to understand why the instance is failing.