Carbon is important in biology for several reasons, one of the most significant being that it can form a variety of carbon skeletons and host functional groups. This property allows carbon to form the backbone of organic molecules such as carbohydrates, lipids, nucleic acids, and proteins, which are essential for life. The ability of carbon to form covalent bonds with other carbon atoms and with a few other elements such as nitrogen, oxygen, and hydrogen gives it a unique versatility in forming complex and diverse molecular structures. In addition, carbon-based molecules are relatively stable, which makes them well-suited for storing and transferring energy within living organisms. While the other options listed such as being a common element on Earth and bonding to only a few other elements are also important, it is the unique ability of carbon to form complex structures and host functional groups that makes it so crucial to biological systems.