The correct answer is A) ammonia. Ammonia is the most common liquefied gas cargo carried by vessels. This is because ammonia is widely used in various industries, such as agriculture, refrigeration, and chemical production. Ammonia has a low boiling point and can be easily liquefied, making it a practical and efficient cargo to transport by ship. The other options are incorrect because: B) Chemical gases are a broader category that includes ammonia, but ammonia is the most common among them. C) Hydrocarbons, such as propane and butane, are also liquefied gases but are less common as cargo compared to ammonia. D) Butane is a specific type of hydrocarbon gas, but it is not as widely used as a cargo compared to ammonia.