Complete the statement: A, T, G, C are _____________ base models.

The correct completion of the statement is that A, T, G, and C are nitrogenous base models. These four letters represent the nucleobases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). Each of these bases plays a critical role in the structure of DNA, as they are the building blocks of nucleotides. Nucleotides consist of a nitrogenous base, a sugar molecule, and a phosphate group, linking to form the backbone of the DNA double helix.

In addition to their role in genetic coding, nitrogenous bases pair in specific ways—adenine pairs with thymine, and guanine pairs with cytosine—this base pairing is fundamental to the process of DNA replication and the transmission of genetic information.

The other options do not accurately describe A, T, G, and C. While "common" may suggest widespread presence, it does not specifically refer to their chemical nature. "Simple sugars" refer to different molecules, specifically carbohydrates, and are part of the structure of nucleotides but not the bases themselves. "Protein bases" is a misnomer; proteins are made of amino acids, not directly of nitrogenous bases,