What characteristic is common to all organic molecules?

The characteristic that is common to all organic molecules is that they contain carbon. Carbon is the fundamental building block of organic chemistry and is unique in its ability to form stable bonds with many elements, most notably hydrogen, oxygen, and nitrogen, which allows for the creation of complex molecules such as proteins, carbohydrates, lipids, and nucleic acids. This carbon-based framework underpins the vast diversity of organic compounds, enabling them to participate in countless biological processes.

The other characteristics mentioned do not universally apply to all organic molecules. For example, while many organic molecules do contain hydrogen, there are also organic compounds that do not strictly adhere to this rule in every instance. The idea that organic molecules are exclusively derived from animals is inaccurate, as many organic compounds can and do come from plant sources and synthetic processes. Lastly, while some organic molecules are water-soluble, such as sugars, others, like fats and oils, are not, demonstrating that solubility is not a common trait across all organic substances. Therefore, the presence of carbon is the defining feature of all organic molecules.