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.
