That depends entirely on the context, as all of these words can mean nearly the same or something completely different in both languages.
Any meaning referring to moneys paid to a person for there cost of living are usually called "Unterhalt," while efforts to keep an object serving its purpose would be either "Wartung" or "Instandhaltung." When talking about living things, the efforts to keep them in good shape is mostly "Pflege" or "Versorgung." "Erhaltung" covers the idea of preserving something for the future, while "Nahrung" and "Ernährung" are the best translation of "sustenance" in regards to maintaining a person's or animal's live force.
HTH
