What element constitutes about 50% of the cellular structure?

The correct answer is carbon because it is a fundamental building block of life and a primary component of many cellular structures. In biological systems, carbon atoms are integral to the formation of organic molecules, including carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are essential for the structure and function of cells.

Carbon's ability to form four covalent bonds with other atoms allows for the creation of complex and diverse organic compounds, which are crucial for cellular functions. Its prevalence in living organisms is reflected in the fact that a significant portion of cellular mass is made up of carbon-containing compounds. Thus, about 50% of the cellular structure is attributed to carbon, making it the correct choice in this context.