Step 1: Understanding the Concept:
Nitrogenous bases in nucleic acids are derivatives of two parent compounds: purine and pyrimidine.
The major purine bases are Adenine and Guanine, while the major pyrimidine bases are Cytosine, Thymine, and Uracil.
Step 2: Key Formula or Approach:
The approach is to recall the molecular structure of the primary nitrogenous bases and inspect them for the presence of an \(-\text{NH}_2\) (amino) group.
Step 3: Detailed Explanation:
Let's examine the chemical structure of each base given in the options:
(A) Adenine: Its chemical name is 6-aminopurine. It has an amino group (\(-\text{NH}_2\)) attached at the C-6 position of the purine ring.
(C) Guanine: Its chemical name is 2-amino-6-oxopurine. It contains an amino group (\(-\text{NH}_2\)) attached at the C-2 position of the purine ring.
(D) Cytosine: Its chemical name is 4-amino-2-oxopyrimidine. It has an amino group (\(-\text{NH}_2\)) attached at the C-4 position of the pyrimidine ring.
(B) Thymine: Its chemical name is 5-methyl-2,4-dioxopyrimidine. The structure consists of a pyrimidine ring with two keto groups (\(=\text{O}\)) at C-2 and C-4, and a methyl group (\(-\text{CH}_3\)) at C-5.
Thymine does not possess an exocyclic amino (\(-\text{NH}_2\)) group attached to its ring.
Step 4: Final Answer:
Thymine is the only base among the choices that lacks an \(-\text{NH}_2\) group.