Purine–pyrimidine identification: which of the following bases is a purine? Choose the correct classification.

Introduction / Context:
Nucleotide bases are grouped into purines and pyrimidines. This classification underlies Watson–Crick pairing rules and influences chemical behavior.

Given Data / Assumptions:

• Purines: two fused rings (adenine, guanine).
• Pyrimidines: single six-membered ring (cytosine, thymine, uracil).
• Options include one purine among pyrimidines.

Concept / Approach:
Identify ring systems: only adenine among the listed choices is a purine. The others are pyrimidines used in DNA (C, T) and RNA (U).

Step-by-Step Solution:
Scan options for purines: adenine qualifies; cytosine, thymine, uracil do not.Select adenine as the correct answer.

Verification / Alternative check:
Chemical structures confirm that adenine is a bicyclic purine, whereas the others are monocyclic.

Why Other Options Are Wrong:
Cytosine, thymine, and uracil are pyrimidines; “all of these are pyrimidines” is false because adenine is included.

Common Pitfalls:
Memorization errors that swap guanine/adenine with pyrimidines; remembering “Cut the Py” (C, U, T are pyrimidines) helps.

Final Answer:
Adenine.