Which one of the following is the principal structural elements of a living cell?

can form a variety of complex molecules that are essential for life:

Bonds
Carbon atoms can form up to four bonds with other atoms, including other carbon atoms. This allows carbon to create long chains and rings, which are the building blocks of biomolecules.

Macromolecules
Carbon is the main component of macromolecules, which are large molecules that are essential for life. These include proteins, nucleic acids (DNA and RNA), carbohydrates, and lipids.