The instrument used for measuring area on maps is called a planimeter. A planimeter is a mechanical device that measures the area of an irregular shape by tracing its outline with a stylus. The device consists of a wheel that rotates as it is moved across the shape, and the rotation of the wheel is directly proportional to the area being measured. This allows for accurate and precise measurements of the area on a map.

Option 2: An eidograph is not a tool used for measuring area on maps. It is a device used for copying drawings or designs.

Option 3: A pantograph is also not used for measuring area on maps. It is a device used for scaling or duplicating drawings.

Option 4: An opisometer is not specifically designed to measure area on maps. It is a device used for measuring distances on maps or diagrams.

Overall, the correct answer is option 1 - planimeter.