## Core Concept
Codons are sequences of three nucleotides that correspond to a specific amino acid or stop signal during protein synthesis. They are fundamental to the process of translating messenger RNA (mRNA) into a protein. The genetic code is nearly universal, with 64 possible codons that code for 20 amino acids and three stop signals.

## Why the Correct Answer is Right
The correct answer, **mRNA (messenger RNA)**, is where codons are primarily found. During transcription, the DNA sequence is transcribed into a complementary RNA molecule, known as mRNA. This mRNA then travels out of the nucleus and into the cytoplasm, where it serves as a template for protein synthesis. The sequence of nucleotides in mRNA is read in codons, which specify particular amino acids.

## Why Each Wrong Option is Incorrect
* **Option A: DNA** - While DNA contains the genetic information that is transcribed into mRNA, the codons themselves are not directly present in DNA; instead, DNA has sequences of nucleotides that are transcribed into codons in mRNA.
* **Option B: tRNA** - Transfer RNA (tRNA) molecules recognize codons on mRNA through their anticodons and bring the corresponding amino acids to the ribosome for protein synthesis. While tRNA interacts with codons, it does not contain them.
* **Option D: rRNA** - Ribosomal RNA (rRNA), along with proteins, makes up ribosomes, which are the sites of protein synthesis. rRNA is involved in the catalysis of peptide bond formation but does not contain codons.

## Clinical Pearl / High-Yield Fact
A key point to remember is that mutations in DNA can lead to changes in the codons of mRNA, potentially altering the amino acid sequence of proteins (missense or nonsense mutations). This can have significant clinical implications, including the development of genetic disorders.

## Correct Answer Line
**Correct Answer: C. mRNA.**