## **Core Concept**
The genetic code is based on sequences of nucleotides (A, C, G, and T) that are translated into amino acids. Normally, codons are sequences of 3 nucleotides that specify particular amino acids. The question posits a scenario where codons are made of 4 nucleotides instead of 3.

## **Why the Correct Answer is Right**
If codons are made of 4 nucleotides instead of 3, the number of possible codons increases. With 4 possible nucleotides (A, C, G, T) at each position, the total number of codons would be $4^4 = 256$. Since there are 20 standard amino acids, the question essentially asks how many different amino acids could be encoded with this expanded codon size. Given that 256 is greater than 20, theoretically, all 20 standard amino acids could still be encoded, plus additional possibilities for stop codons or other signals. However, the question seems to focus on the maximum variety of amino acids that could be specified, not accounting for the specifics of the genetic code's degeneracy or stop codons.

## **Why Each Wrong Option is Incorrect**
- **Option A:** $4^3 = 64$, which corresponds to the current genetic code's possibilities with 3-nucleotide codons, not 4.
- **Option B:** $4^2 = 16$, which is less than the number of standard amino acids and doesn't account for 4-nucleotide codons.
- **Option C:** This option is not provided but based on calculations, we can assess its validity.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the standard genetic code uses codons of 3 nucleotides to encode 20 amino acids, with 64 possible codons due to $4^3$. This results in some amino acids being encoded by more than one codon (synonymous codons). The expansion to 4 nucleotides per codon dramatically increases the potential coding capacity.

## **Correct Answer:** D. 256.