The correct answer is option 4, Carbon. Carbon is known for its ability to form a wide variety of compounds, making it one of the most versatile elements in chemistry. It has the unique property of being able to form covalent bonds with other elements, including itself. The ability of carbon atoms to bond together in long chains or rings allows for the formation of complex organic molecules.

Option 1, Oxygen, is also a highly reactive element and forms many compounds. However, carbon surpasses oxygen in the number of compounds it can form.

Option 2, Hydrogen, is the most abundant element in the universe and forms a large number of compounds as well. However, carbon forms a greater variety of compounds than hydrogen.

Option 3, Chlorine, is also a highly reactive element, but it forms fewer compounds compared to carbon.

In summary, while all of these elements can form compounds, carbon is the element that forms the highest number and variety of compounds, making it the correct answer.