**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 code is composed of sequences of nucleotides that are read in triplets, called codons. Each codon corresponds to a specific amino acid or a stop signal, which terminates protein synthesis.

**Why the Correct Answer is Right**
Nonsense codons, also known as stop codons, are specific sequences of three nucleotides in messenger RNA (mRNA) that signal the termination of protein synthesis. They are called "nonsense" because they do not code for any amino acid. The three nonsense codons in the genetic code are UAA (ochre), UAG (amber), and UGA (opal). These codons are recognized by release factors, which cause the ribosome to release the completed polypeptide chain from the tRNA molecules and terminate protein synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** UAC is actually a codon for the amino acid tyrosine. It is not a nonsense codon.
**Option B:** UAG is actually one of the nonsense codons, but it is not the only correct answer given in the options.
**Option C:** UGU is a codon for the amino acid cysteine. It is not a nonsense codon.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that nonsense mutations can lead to genetic disorders by causing premature termination of protein synthesis. This can result in a truncated or non-functional protein, leading to various diseases.

**Correct Answer:** B. UAG (amber)