**Core Concept**
The genetic code is based on a 3-nucleotide sequence, known as a codon, which specifies one of the 20 amino acids used in protein synthesis. This 3-nucleotide code allows for a vast number of possible combinations, enabling the encoding of a diverse range of amino acids.

**Why the Correct Answer is Right**
If a 4-nucleotide sequence is used to code for an amino acid, the number of unique amino acids that could be coded would be determined by the number of possible combinations of these 4 nucleotides. Since each nucleotide can be one of four possible bases (A, C, G, or T), the total number of unique 4-nucleotide combinations would be 4^4 = 256. However, not all of these combinations would be valid codons, as some would code for stop signals or other nonsense sequences. Assuming that each 4-nucleotide sequence is a valid codon, we can estimate the number of unique amino acids that could be coded by dividing the total number of combinations by the number of amino acids per codon. However, given that there are 20 amino acids in the standard genetic code, we can simply take the total number of combinations as the number of unique amino acids, since each combination would theoretically correspond to a unique amino acid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, as the question only lists three options.
**Option B:** This option is not provided, as the question only lists three options.
**Option C:** This option is not provided, as the question only lists three options.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the standard genetic code uses a 3-nucleotide sequence to specify one of the 20 amino acids. Any deviation from this code, such as using a 4-nucleotide sequence, would result in a significant increase in the number of possible codons and, consequently, the number of unique amino acids that could be coded.

**Correct Answer:** C. 256.