A watchmaker relies on a magnifying lens to see the fine details of a watch’s components ; option [B]. This tool is crucial for magnifying small parts like gears, springs, and screws, enabling precise inspection and intricate repair work. Unlike a compound microscope (option C), which is designed for viewing microscopic objects with high magnification, a magnifying lens provides sufficient enlargement for detailed examination of watch parts without requiring such extreme magnification. Option A, a photo camera, is used for capturing images rather than for direct visual inspection. Option D, a telescope, is designed for viewing distant objects and would not be suitable for examining close-up details of a watch. Therefore, a magnifying lens (option B) is the most appropriate tool for a watchmaker to accurately work on the delicate mechanisms within a watch.