Identify what cytosine is

Cytosine is one of the nitrogenous bases present in nucleic acids. It is a pyrimidine base.

Recall the structure of a pyrimidine ring

A pyrimidine ring is a six-membered aromatic ring containing two nitrogen atoms at positions 1 and 3.

Recall the substituents in cytosine

Cytosine is 4-amino-2-oxo pyrimidine.

That means on the pyrimidine ring:

• at position 2: there is a keto group $(=O)$
• at position 4: there is an amino group $(-NH_2)$

So the correct structure must be: $\text{2-oxo-4-aminopyrimidine}$

General comparison with common bases

• Cytosine: one keto group and one amino group on a pyrimidine ring
• Uracil: two keto groups
• Thymine: two keto groups + one methyl group
• Adenine/Guanine: purine bases (fused rings), so not cytosine

Match with options

Since the correct cytosine structure is the pyrimidine ring having: $\boxed{\text{one } =O \text{ at C-2 and one } -NH_2 \text{ at C-4}}$ the correct option is C.

Comparison with stored answer

• Derived answer: C
• Stored correct answer: C

Hence, they match.