Which contrast agent decreases the tissue density in a body area when administered?

The correct choice is carbon dioxide, which is classified as a negative contrast agent. When administered, it decreases tissue density in the specific area where it is used, thus enhancing the contrast in imaging studies. This is particularly useful in enhancing the visibility of structures during imaging procedures like gastrointestinal studies or in certain vascular imaging techniques.

Negative contrast agents, such as carbon dioxide, are characterized by their ability to provide a contrast effect by decreasing the density of the tissues they fill, which subsequently allows nearby structures to appear more prominent on the images. This is in contrast to positive contrast agents like barium, gadolinium, and iodine, which increase the tissue density and enhance visibility by appearing brighter than surrounding tissues on imaging.