**Core Concept**
Adenine and guanine are purine bases, but only adenine contains an amino group at carbon 6, playing a crucial role in forming hydrogen bonds with thymine in DNA and with uracil in RNA. This unique chemical property is vital for the structure and function of nucleic acids.

**Why the Correct Answer is Right**
Adenine has an amino group (-NH2) attached to the 6th carbon atom of the purine ring. This amino group forms a hydrogen bond with the keto group (-C=O) of thymine in DNA, and with the keto group of uracil in RNA, contributing to the specificity and stability of these base pairs. The presence of the amino group at carbon 6 is a defining characteristic of adenine, distinguishing it from other purine bases like guanine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll proceed with the remaining options.
**Option B:** Guanine does not have an amino group at carbon 6; instead, it has a keto group (-C=O) at this position.
**Option C:** Hypoxanthine, a purine base, lacks an amino group at carbon 6 and is often considered a product of adenine deamination.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Remember that the unique chemical structure of adenine, particularly the presence of an amino group at carbon 6, is essential for its role in forming specific base pairs with thymine in DNA and uracil in RNA.

**Correct Answer: A. Adenine**