When considering the fundamental characteristics of organic compounds, the most crucial element is Carbon (C). All organic compounds, by definition, contain carbon atoms, which serve as the backbone for these molecules. While many organic compounds also contain other elements like Nitrogen (N), Calcium (Ca), or Sodium (Na), the presence of carbon is what classifies a compound as organic.
Therefore, the correct answer is A) C. Without carbon, a compound cannot be classified as organic. This definition is a cornerstone of organic chemistry, emphasizing the unique role that carbon plays in forming the vast diversity of organic substances ranging from simple hydrocarbons to complex biomolecules.