Organic molecules primarily consist of atoms of carbon. Therefore, the correct answer is a) carbon.
To elaborate, organic chemistry is fundamentally centered around carbon-containing compounds. Carbon’s unique ability to form stable bonds with a variety of elements, including itself, allows for the creation of complex and diverse structures. While other elements such as hydrogen, oxygen, and nitrogen frequently accompany carbon in organic molecules, the defining feature of these compounds is the presence of carbon atoms. Hence, even though options b, c, and d include elements commonly found in organic compounds, they don’t encompass the essential defining characteristic of organic molecules like option a does.