d. Nucleotides link together to form a nucleic acid. Amino acids link together to form a protein.
Explanation:
A nucleotide is a bio-molecule which has a sugar phosphate backbone in which a sugar (deoxyribose / ribose sugar) and a phosphate group is attached to each other alternately through phosphodiester bond and a nitrogenous base is attached to the sugar molecule with the help of N- Glycosidic bond.
In case of nucleic acids (DNA and RNA), nucleotides are the monomer units and when these monomeric units join each other to form a chain of nucleotides they are able to create these polymers known as nucleic acids.
Proteins on the other hand are another class of bio-molecules which are the polymers of monomeric units known as amino acids. In proteins, various amino acids are attached to each other with the help of peptide bond.
Animals and plants need nitrogen to build amino acids in proteins, which are the building blocks of life. Unlike oxygen, nitrogen cannot be absorbed directly from the air by animals and plants.
