**Core Concept**
The genetic code is a set of rules used by living cells to translate information encoded within genetic material (DNA or RNA) into proteins. The genetic code consists of 64 possible codons, which are sequences of three nucleotides that specify particular amino acids or stop signals during protein synthesis. Nonsense codons, also known as stop codons, are a subset of these codons that signal the termination of protein synthesis.

**Why the Correct Answer is Right**
Nonsense codons do not encode any amino acids but instead serve as stop signals to terminate protein synthesis. There are three nonsense codons in the genetic code: UAA, UAG, and UGA. These codons are recognized by release factors, which bind to the ribosome and cause the release of the completed protein from the ribosome. The presence of nonsense codons in a gene's sequence can lead to the production of truncated or aberrant proteins, which can have significant consequences for cellular function and disease.

**Why Each Wrong Option is Incorrect**
**Option A:** 20 - This is incorrect because nonsense codons do not encode amino acids, so they are not part of the 20 standard amino acids used in protein synthesis.
**Option B:** 3 - This is incorrect because nonsense codons are a subset of the 64 possible codons in the genetic code, not the number of codons in general.
**Option C:** 64 - This is incorrect because while there are 64 possible codons, not all of them are nonsense codons.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that nonsense mutations, which result from the substitution of a nonsense codon for a sense codon, can lead to severe genetic disorders, including certain types of muscular dystrophy and cystic fibrosis.

**Correct Answer:** C. 3.