Which of the following is true about organic compounds?

Organic compounds are primarily characterized by their carbon content, and most of them contain hydrogen as well. This is important because the presence of carbon (and often hydrogen) is what distinguishes organic compounds from inorganic ones. Many organic compounds form the basis of biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids, where hydrogen plays a critical role in their structure and stability.

While it's true that many organic compounds do contain hydrogen, they can also include other elements such as oxygen, nitrogen, sulfur, and phosphorus, thereby facilitating a vast diversity of structures and functions.

The other statements about organic compounds are inaccurate because organic compounds can exist without hydrogen, as seen in some exceptions like carbon allotropes (e.g., carbon black). They do not have a universally fixed structure, as structures can vary widely among different organic compounds based on configurations and functional groups. Lastly, while some organic compounds might be gaseous at room temperature, many are solid or liquid, indicating that they are not limited to a gaseous state.