**Core Concept**
A mutation in the codon is a type of genetic variation that alters the sequence of nucleotides in DNA, leading to a change in the coded amino acid. This change can result in the substitution of a different amino acid, a stop codon, or a start codon, ultimately affecting protein structure and function.

**Why the Correct Answer is Right**
This type of mutation is known as a **missense mutation**. It occurs when a single nucleotide change in the codon results in the substitution of a different amino acid. For example, if the codon for the amino acid alanine (GCT) is mutated to GAT, the resulting protein will have a different amino acid at that position. This change can have significant effects on protein function, leading to various diseases.

**Why Each Wrong Option is Incorrect**
* **Option A:** A nonsense mutation is a type of mutation that results in a premature stop codon, leading to a truncated protein. This is not the correct answer because it does not involve a change in the coded amino acid.
* **Option B:** A silent mutation is a type of mutation that does not change the coded amino acid. This is not the correct answer because it does not involve a change in the coded amino acid.
* **Option D:** A frameshift mutation is a type of mutation that changes the reading frame of the genetic code, leading to a completely different amino acid sequence. This is not the correct answer because it is a different type of mutation altogether.

**Clinical Pearl / High-Yield Fact**
Missense mutations can be caused by a single nucleotide change in the DNA sequence, and they can have significant effects on protein function. This type of mutation is a common cause of genetic diseases, including sickle cell anemia and cystic fibrosis.

**Correct Answer:** C.