Final answer:
Both receiving a DUI and having a client file a complaint with the state licensing board should be immediately reported to the BACB, as they mandate self-reporting of legal charges or disciplinary actions related to one's practice.
Step-by-step explanation:
The situation that should be immediately reported to the Behavior Analyst Certification Board (BACB) is both receiving a DUI (Driving Under the Influence) and having a client file a complaint against you with the state licensing board. According to the BACB's ethical guidelines, certificants are required to self-report any legal charges or disciplinary actions that may affect their ability to practice. This includes any criminal charges relating to alcohol or drugs, such as a DUI, as well as formal complaints or sanctions from regulatory bodies.