**Core Concept**
The question is testing the concept of the **genetic code** and how multiple **codons** can code for the same **amino acid** due to the degeneracy of the genetic code. This is a fundamental principle in **molecular biology** and **genetics**.

**Why the Correct Answer is Right**
The correct answer is related to the **degeneracy of the genetic code**, where more than one codon can specify the same amino acid. This occurs because there are 64 possible codons but only 20 amino acids, resulting in some amino acids being encoded by more than one codon. The specific mechanism involves the **wobble hypothesis**, which explains how a single **tRNA** molecule can recognize more than one codon.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the concept of codon degeneracy.
**Option B:** This option is also incorrect as it does not explain why multiple codons code for the same amino acid.
**Option C:** This option is incorrect because it is unrelated to the genetic code and codon degeneracy.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the degeneracy of the genetic code allows for some **genetic flexibility** and **mutation tolerance**, as multiple codons can encode the same amino acid. This is an important concept in understanding **genetic diseases** and **evolutionary biology**.

**Correct Answer:** D. Degeneracy of genetic code.