Final answer:
Sodium is the least likely to bond with carbon as a functional group in biological systems, as it does not form covalent bonds in organic molecules and typically exists as a free ion or forms ionic bonds. The correct option is d.
Step-by-step explanation:
The question asks which of the following is least likely to bond with carbon as a functional group in biological systems: carbonyl, hydroxyl, phosphate, or sodium.
In biological molecules, functional groups such as hydroxyl, carbonyl, and phosphate are common. These functional groups are a part of organic compounds and play an essential role in their reactivity and function.
Sodium, on the other hand, is an inorganic ion that does not form covalent bonds as part of a functional group in organic molecules. It typically exists as a free ion in biological systems or forms ionic bonds with other elements. Therefore, sodium is the least likely to bond with carbon to form a functional group in biological molecules.
Furthermore, the carbonyl and hydroxyl groups are polar, making them hydrophilic, while the phosphate group is part of the building blocks of proteins. Option d. is the correct one.