The correct answer is A. Carbon.
Carbon is the only element that is found in all organic compounds. Organic compounds are compounds that contain carbon. They are found in living things, such as plants and animals, and in non-living things, such as oil and coal.
Carbon is a very versatile element. It can form a large number of different compounds, more than any other element. This is because carbon atoms can bond to each other in a variety of ways. They can form single bonds, double bonds, and triple bonds. They can also form chains, rings, and branched structures.
The ability of carbon to form a large number of different compounds is what makes organic chemistry so complex. Organic chemistry is the study of carbon compounds. It is a very important field of study, as organic compounds are essential for life.
The other options are incorrect because they are not found in all organic compounds. Calcium is a metal that is found in bones and teeth. Nitrogen is a gas that is found in the air. Oxygen is a gas that is found in the air and water.