The correct answer is A. carbon and hydrogen.
Carbon and hydrogen are the two elements that can form the largest number of compounds. This is because they have a very strong chemical bond, which allows them to form a wide variety of molecules. Carbon and hydrogen are also the two most abundant elements in the universe, which means that they are readily available for forming compounds.
The other options are also elements that can form compounds, but they do not form as many compounds as carbon and hydrogen. Nitrogen and oxygen can form a large number of compounds, but they are not as abundant as carbon and hydrogen. Sulfur can form a few compounds, but it is not as reactive as carbon and hydrogen.
In conclusion, the correct answer is A. carbon and hydrogen.