Nucleotide Structure and Base Properties

Steps:

• Statement 1 is correct: Nucleotides in RNA contain ribose sugar, so the carbohydrate could be ribose.
• Statement 2 is correct: Cytosine is a nitrogenous base, containing nitrogen atoms in its ring structure.
• Statement 3 is correct: In DNA and RNA, cytosine forms three hydrogen bonds with guanine during base pairing.
• Statement 4 is incorrect: Cytosine is a pyrimidine (single-ring base), not a purine (double-ring base).

Why A is correct:

• A selects statements 1, 2, and 3, which accurately describe the nucleotide's possible sugar, base composition, and pairing rules per nucleic acid structure.

Why the others are wrong:

• B includes incorrect statement 4 about cytosine being a purine.
• C includes incorrect statement 4 about cytosine being a purine.
• D includes incorrect statement 4 about cytosine being a purine.

Final answer: A