**Core Concept**
The question is testing the student's understanding of messenger RNA (mRNA) structure and its role in protein synthesis. mRNA is a single-stranded RNA molecule that carries genetic information from DNA to the ribosome, where it is translated into a protein. The tail of mRNA refers to a specific sequence of nucleotides that plays a crucial role in its stability and translation efficiency.

**Why the Correct Answer is Right**
The tail of mRNA is known as the poly-A tail, which is a sequence of adenine (A) nucleotides added to the 3' end of the mRNA molecule. This poly-A tail helps to protect the mRNA from degradation by exonucleases and also facilitates its transport out of the nucleus and into the cytoplasm. The poly-A tail is recognized by specific proteins that bind to it, thereby stabilizing the mRNA and promoting its translation into a protein.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the 5' cap, not the 5' end, is a modified guanosine nucleotide that protects the mRNA from exonucleases. The 5' cap is a distinct feature of mRNA, but it is not the tail.
* **Option B:** This is incorrect because the 3' untranslated region (UTR) is the sequence of nucleotides at the 3' end of the mRNA, but it is not the tail. The 3' UTR contains regulatory elements that control mRNA stability and translation, but it is not a sequence of adenine nucleotides.
* **Option D:** This is incorrect because the poly-C tail is not a characteristic feature of mRNA. While some RNAs, such as some viral RNAs, may have a poly-C tail, this is not a common feature of eukaryotic mRNAs.

**Clinical Pearl / High-Yield Fact**
The poly-A tail of mRNA is a key feature that distinguishes it from other types of RNA, such as transfer RNA (tRNA) and ribosomal RNA (rRNA). Understanding the structure and function of the poly-A tail is essential for grasping the mechanisms of gene expression and protein synthesis.

**Correct Answer:** C.