Which of the following is NOT one of the important surveillance tools for private investigators?

The correct response highlights the distinction between typical surveillance tools and devices used primarily for communication and convenience. While smartphones can certainly be utilized during investigations for various tasks—such as taking notes, communicating with clients, or accessing information—they are not specialized surveillance equipment. Surveillance tools, on the other hand, typically include devices that enhance an investigator's ability to gather visual evidence discreetly and effectively.

Digital video cameras, binoculars, and SRL (Single Lens Reflex) cameras are specifically designed for capturing images or video from a distance, enabling private investigators to observe subjects without being detected. Each of these tools enhances the investigator's observational capabilities, which is crucial in surveillance work. Therefore, while smartphones are valuable in an investigator's toolkit, they do not fit within the category of essential surveillance tools like the others mentioned.