**Core Concept**
A codon is a sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule. It specifies one of the twenty amino acids or a stop signal during protein synthesis. Codons are crucial for translating the genetic information encoded in the DNA or RNA into a specific sequence of amino acids.

**Why the Correct Answer is Right**
A codon is composed of three nucleotides, which are adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA, but thymine (T) in DNA. However, the question specifically asks about nucleic acid, and the answer choice should be a nucleic acid that is not found in a codon. The correct answer is **uracil** because it is a nucleotide base that is found in RNA, but not in DNA. In DNA, the base that replaces uracil is thymine.

**Why Each Wrong Option is Incorrect**
* **Option B:** This option is not a nucleotide base, so it cannot be the correct answer.
* **Option C:** This option is not related to the composition of a codon, so it is incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the genetic code is read in triplets, with each codon specifying one of the twenty amino acids or a stop signal. The genetic code is almost universal, with some variations in mitochondria.

**Correct Answer: C. Uracil.**