Cytosine's Structural Features

Steps:

• Classify cytosine as a pyrimidine base, which has a single six-membered ring.
• Confirm cytosine's molecular formula (C4H5N3O) includes three nitrogen atoms.
• Eliminate options with double rings (purines like adenine) or incorrect nitrogen counts.
• Select the option matching single ring, three nitrogens, and pyrimidine type.

Why C is correct:

• Cytosine is a pyrimidine with a single ring and three nitrogen atoms, as per its formula C4H5N3O.

Why the others are wrong:

• A: Double ring describes purines, not cytosine.
• B: Two nitrogens fits thymine, not cytosine.
• D: Purine classification is incorrect for cytosine.

Final answer: C