All organic substances contain which element?

All organic substances are defined by the presence of carbon. In organic chemistry, carbon is the central element that forms the backbone of organic molecules due to its unique ability to form stable bonds with a variety of other elements, including hydrogen, oxygen, nitrogen, and others. This versatility allows for the complexity and diversity of organic compounds, which include hydrocarbons, carbohydrates, proteins, and more.

While nitrogen, hydrogen, and oxygen can be present in organic substances, they are not defining characteristics. Carbon's tetravalent nature enables it to bond in chains and rings, creating the structural framework necessary for life and various biochemical processes. Therefore, the essence of organic chemistry is fundamentally linked to the presence of carbon, making it the correct answer to the question.