The answer is D.
Organic molecules are usually classified as those that contain carbon as the main constituent, except in some cases such as carbon dioxide. From the options, iron oxide has the formula FeO or Fe2O3 (depending on the valency iron exhibits), but neither contains carbon. Moreover, oxygen gas consists of only oxygen and water consists of oxygen and hydrogen. The remaining compound, sucrose, is a carbohydrate found in the bodies of many organisms. It contains a carbon backbone which allows it to be classified as an organic molecule.