**Core Concept**
The 7-methyl guanosine cap, also known as the 5' cap, is a modified nucleotide residue that protects the 5' end of eukaryotic mRNA from degradation. This modification is essential for the stability and translation of mRNA.

**Why the Correct Answer is Right**
The 7-methyl guanosine cap is added to the 5' end of mRNA during its synthesis in the nucleus. This process involves the enzyme guanylyltransferase, which catalyzes the transfer of a guanine nucleotide to the 5' end of the growing mRNA chain. The resulting 5' cap is then methylated by the enzyme methyltransferase, resulting in the formation of 7-methyl guanosine. The 5' cap plays a crucial role in protecting the mRNA from exonuclease degradation, facilitating its translation, and regulating its stability.

**Why Each Wrong Option is Incorrect**
**Option B:** t-RNA (transfer RNA) does not have a 7-methyl guanosine cap. Instead, t-RNA is modified with other nucleotides, such as dihydrouridine and pseudouridine, which are essential for its function in protein synthesis.
**Option C:** r-RNA (ribosomal RNA) is a component of the ribosome, which is involved in protein synthesis. While r-RNA is modified with nucleotides such as pseudouridine, it does not have a 7-methyl guanosine cap.
**Option D:** DNA (deoxyribonucleic acid) does not have a 7-methyl guanosine cap. DNA is a double-stranded molecule that is not modified with a 5' cap.

**Clinical Pearl / High-Yield Fact**
The 7-methyl guanosine cap is a critical modification that distinguishes eukaryotic mRNA from prokaryotic mRNA. The absence of a 5' cap in prokaryotic mRNA is one of the key differences between eukaryotic and prokaryotic translation.

✓ Correct Answer: A. M-RNA