The correct answer is a device that writes down (records) the movements of the earth. A seismograph is a device specifically designed to measure and record seismic activity, such as earthquakes and tremors. It detects and records the vibrations and movements of the earth's surface. The other options mentioned in the question, such as recording the weight of furniture, the length of a road, or the depth of a lake, are not related to the function of a seismograph.