What device is used to accurately position a lens and mark it with reference lines for lens blocking?

The correct answer is based on the specific task of marking a lens with reference lines during the blocking process. A marker is an essential tool for this purpose, as it allows the optician to draw precise lines on the lens that indicate the orientation and position for mounting it in the blocking device. This is critical because accurate placement and alignment ensure that the finished lenses are fabricated correctly and provide optimal vision for the wearer.

Other options, while they may have relevance in lens preparation and fitting, do not specifically address the task of marking the lens for blocking. A lens protractor is typically used for measuring angles, which is not directly related to marking lenses. A template serves as a guide for shaping or cutting lenses but does not mark lens positions. A blocking device, on the other hand, is the apparatus used to hold the lens in place during the grinding process but does not perform the marking itself. Therefore, the marker stands out as the most appropriate tool for accurately positioning and marking lenses for blocking.