Uracil as a pyrimidine base in RNA

Steps:

• Classify nucleic acid bases: purines have two rings, pyrimidines have one.
• Identify uracil as a pyrimidine base, replacing thymine in RNA.
• Confirm uracil's structure: single six-membered ring.
• Match to options: not a purine with single ring fits C.

Why C is correct:

• Pyrimidines, including uracil, are defined by their single-ring heterocyclic structure, unlike purines' fused double rings.

Why the others are wrong:

• A: Uracil is a pyrimidine, not a purine.
• B: Uracil is a pyrimidine and is found in RNA.
• D: Uracil has a single ring as a pyrimidine.

Final answer: C