Which of the following is true about organic compounds?

Organic compounds are primarily defined by their carbon content, and most of them contain hydrogen as well. While it is true that carbon is the backbone of organic chemistry, hydrogen is often found bonded to carbon atoms in various configurations, which is essential for the structure and reactivity of these compounds.

For instance, hydrocarbons, which are simple organic compounds, consist entirely of hydrogen and carbon. Moreover, many functional groups, fundamental to organic chemistry, also contain hydrogen, reinforcing the idea that hydrogen is a critical component of organic compounds.

In contrast, there are exceptions to the notion of a "fixed structure" in organic compounds, as many can exist in various forms (isomers) or can undergo reactions changing their structure. Additionally, organic compounds can exist in solid, liquid, or gaseous states, depending on factors such as molecular weight and intermolecular forces, so the assertion that they are always gaseous is incorrect. Lastly, organic compounds are characterized by their carbon content, making the statement about their lack of carbon false. Overall, the requirement for hydrogen in most organic compounds underscores its significance in organic chemistry, solidifying the correctness of the answer provided.