Carbon exists naturally in the combined state as all of the options provided: graphite, coal, diamond, and wood. Graphite is a form of carbon that is used in pencils, while coal is a fossil fuel that is formed from the remains of ancient plants and animals. Diamond is a precious gemstone made entirely of carbon atoms arranged in a crystal lattice structure, while wood is a natural composite material made up of cellulose, hemicellulose, and lignin, all of which contain carbon. Therefore, the correct answer is that carbon exists naturally in the combined state as graphite, coal, diamond, and wood.