Which of the following should be done immediately after discovering body fluids?

The most appropriate action to take immediately after discovering body fluids is to protect the area. This step is crucial as it helps to preserve the scene for investigation and prevents contamination of evidence. By securing the area, you ensure that any potential evidence related to the investigation remains intact, which is vital for any future forensic analysis or legal proceedings. Protecting the scene also serves to keep unauthorized personnel away, which could disrupt the investigation.

Taking immediate steps to isolate and control the area allows law enforcement officers to safely manage the scene while awaiting further assistance or forensic teams who may need to process the evidence. Other options, such as waiting for backup or leaving the scene untouched, would delay necessary precautions that could aid in a thorough investigation. Documenting the scene is indeed important but typically comes after ensuring the area is secured and protected.