In nucleic acids, which RNA base is complementary to thymine ($T$) by standard base-pairing rules?

• A. Uracil ($U$)
• B. Adenine ($A$)
• C. Cytosine ($C$)
• D. Guanine ($G$)

Answer: (B)

Verified step by step guidance

1. Recall the standard base-pairing rules in nucleic acids: in DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).
2. Understand that RNA differs from DNA in that thymine (T) is replaced by uracil (U).
3. In RNA, adenine (A) pairs with uracil (U) instead of thymine, but when considering the complementary base to thymine (T) specifically, adenine (A) is the correct partner.
4. Therefore, when asked which RNA base is complementary to thymine (T) by standard base-pairing rules, identify the base that pairs with thymine in DNA, which is adenine (A).
5. Conclude that adenine (A) is the RNA base complementary to thymine (T) according to standard base-pairing rules.