RNA base pairing — In cellular RNA, uracil (U) base-pairs with which nucleotide during Watson–Crick pairing?

Introduction:
Canonical base pairing underlies the accuracy of transcription and translation. In RNA, uracil replaces thymine and forms specific hydrogen bonds with its complementary base. This question tests recall of standard pairing rules in RNA.

Given Data / Assumptions:

• RNA uses U where DNA uses T.
• Watson–Crick rules apply to standard RNA duplexes.
• Wobble pairing can occur at tRNA anticodons but is not the canonical rule.

Concept / Approach:
Apply Watson–Crick pairing: A pairs with U in RNA (analogous to A–T in DNA), and G pairs with C. Although inosine can wobble pair in tRNA, it is not the universal canonical partner for U in standard duplex RNA.

Step-by-Step Solution:

Verification / Alternative check:
Structures of RNA duplexes and mRNA–tRNA interactions confirm A–U hydrogen-bonding patterns analogous to A–T in DNA.

Why Other Options Are Wrong:

Common Pitfalls:
Overgeneralizing wobble rules; forgetting that U replaces T in RNA.

Final Answer:
Adenine.