Idon't understand the question it says any organism that has a true nucleus is in the domain:

b. the carbon cycle ensures living things have a source of hydrogen

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.