The correct answer is A) they do not easily jam. Chinese stoppers are widely used on merchant vessels because they are safe and easy to apply, and they do not easily jam. This is an important property of Chinese stoppers, as jamming could create a safety hazard on a vessel. The other options are not the primary attributes associated with Chinese stoppers. They may be lighter than standard stoppers, but this is not their main advantage. They can hold the strain without the aid of a seaman, but this is not a unique feature. Lastly, they do not necessarily increase the breaking strain of the line, which is more dependent on the line's material and construction.