Answer:
a) Carbon.
Step-by-step explanation:
The organic chemistry is normally based on the chemical elements Carbon, Hydrogen,oxygen, sulphur and Nitrogen. Sometimes a mixture or compound of one or more of these will be found such as (carbohydrates, fats, proteins). The varieties of organic molecules are based on these atoms, with carbon as the skeleton of the organic molecule.
Hence out of the four options given we find that though nitrogen, sulphur are also common in organic the most common is the first option
a) Carbon.