Annie is using a microscope to look at a single-celled organism. She sees chloroplasts, a nucleus and other membrane-bound organelles, and a flagellum that helps the cell swim back and forth on the slide.

She estimates its size to be over 100 micrometers (0.1 mm). Which of the following characteristics is most important when determining if an organism should be classified in the Eukarya Domain?

A. The presence of a chloroplast.

B. The presence of a nucleus.

C. The presence of a mitochondria.

D. The presence of a flagellum.