The correct answer among the choices listed above is option C. The amount of carbon dioxide in the atmosphere has <span>increased</span> since industrialization began in the nineteenth century. This is because more carbon dioxide is emitted by machines in plants and by the vehicles.
I choose letter "C. increased" for this item. The increase of carbon dioxide in the atmosphere is caused by many factors but is primarily due to emissions from industrial plants, great increase in number of vehicles, and many other activities.