The correct response is: They are multicellular.
However, it's important to note that while many organisms in domain Eukarya are multicellular (like plants and animals), there are also unicellular organisms (like protists and some fungi) in this domain. Therefore, a more accurate general characteristic is that they have membrane-bound cell structures (eukaryotic cells). If you need to choose one from the given options based on the context of Eukarya as a whole, the "multicellular" characteristic could be assumed, but remember the diversity in this domain.