Answer: The correct answer is-
B) guanine.
DNA ( deoxyribonucleic acid) is considered as the genetic material of living organisms. It is made up of small units called nucleotides. A nucleotide is hat is composed of -
1)nitrogenous base, which can be Adenine, Thymine, Guanine, Cytosine
2) Phosphate group, 3) Deoxyribose sugar.
In a DNA molecule, Adenine pairs with Thymine, and Guanine pairs with Cytosine as per the Watson and Crick base pairing.
If the green nitrogen base in the DNA model represents adenine, this indicates that thymine will also be green ( as it pairs with adenine).
Thus, the blue nitrogenous base will be represented by Guanine.