Which hardware component is essential for a complete Milestone video surveillance system?

The essential hardware component for a complete Milestone video surveillance system is the camera. Cameras serve as the primary input devices that capture video footage, which is then processed and managed by the Milestone software. They are critical for monitoring and recording activities in the areas of interest, enabling effective surveillance.

While monitors, routers, and speakers can enhance the functionality and user experience of a surveillance system—for instance, by allowing for video playback, remote access, and audio alerts—they do not perform the fundamental role of video capture. The camera is indispensable because, without it, there would be no video data for the system to function. Thus, the correct answer emphasizes the foundational role that cameras play in video surveillance technology.