The correct answer is C) Only when it is overhauled. Mechanical marine chronometers are designed to run continuously and maintain a consistent rate over long periods of time, often for years. They do not need to be reset or adjusted on a regular basis, such as at the start of each voyage or whenever the error exceeds a certain threshold. The chronometer should only be reset when it is taken for routine maintenance and overhaul, which is typically recommended every 1-2 years. As long as the chronometer continues to maintain a stable rate, there is no need to reset it during normal operations. The other answer choices are incorrect because they suggest unnecessary or improper handling of the chronometer, which could actually cause more problems and inaccuracy if done too frequently.