What is a key characteristic of organic compounds?

The correct choice is that organic compounds contain carbon atoms. Carbon is fundamental to the structure of organic molecules, which can form stable bonds with many other elements due to its tetravalency. This property allows for the formation of a wide variety of structures, ranging from simple hydrocarbons to complex macromolecules like proteins and nucleic acids.

When we consider the other options, the notion that organic compounds have a simple molecular structure is misleading. While some organic compounds can indeed be straightforward in their composition, many are quite complex due to the presence of multiple functional groups and varied bonding arrangements. Moreover, the assertion that organic compounds are all gases at room temperature is inaccurate, as they can exist in all physical states: solids, liquids, and gases. Hence, while some organic compounds are gases (like methane), many, such as glucose and fatty acids, are solid or liquid at room temperature. Lastly, while the statement about the complexity of their molecular structure can sometimes apply, it is not a defining characteristic of organic compounds as a whole. The defining trait remains the presence of carbon, which is central to organic chemistry.