Select the best word for the blank in the following sentence.
Cara chose to study Jupiter, Saturn, _________ for her science project.

Both “Neptune” and “Sun” are capitalized in this sentence because they are proper nouns. Proper nouns are always capitalized and they include name-specific people, places, organizations, and things.
Note: If you were talking about suns, in general, of which there are many in the universe, you would not capitalize the word sun. Example: “Each solar system revolves around a sun.” Then, you would not be talking about any particular Sun. But, when you use the article "the" before the sun and it is mentioned along with other celestial bodies, it indicates one particular sun and that is capitalized.