**Core Concept**
In prokaryotic cells, protein synthesis initiates with a specific amino acid that distinguishes it from eukaryotic cells. This process involves the translation of messenger RNA (mRNA) into a polypeptide chain. The correct amino acid is crucial for the initiation of translation and sets the stage for the elongation process.

**Why the Correct Answer is Right**
The initial amino acid in prokaryotic protein synthesis is formyl-methionine (fMet). This unique amino acid is attached to the first transfer RNA (tRNA) molecule that binds to the small subunit of the ribosome during initiation. Formyl-methionine is formed by the enzyme methionyl-tRNA formyltransferase, which adds a formyl group to methionine. The formyl group is essential for the recognition of fMet by the ribosomal complex, allowing translation to commence. This step is specific to prokaryotic cells and is not seen in eukaryotic cells, where methionine is the initial amino acid.

**Why Each Wrong Option is Incorrect**
**Option A:** Arginine is not involved in the initiation of protein synthesis in prokaryotes. It plays a different role in the cell and is not recognized by the ribosome as the initial amino acid.
**Option B:** Methionine is the initial amino acid in eukaryotic protein synthesis, not prokaryotic. In prokaryotes, methionine is added after the formyl group is removed, making it the second amino acid.
**Option D:** Alanine is not involved in the initiation of protein synthesis in prokaryotes. It is a common amino acid found in many proteins but is not recognized by the ribosome as the initial amino acid.

**Clinical Pearl / High-Yield Fact**
Remember that formyl-methionine is the unique initial amino acid in prokaryotic cells, which distinguishes it from eukaryotic cells where methionine is the initial amino acid. This difference is crucial for understanding the mechanisms of protein synthesis in different types of cells.

**✓ Correct Answer: C. Formyl-methionine**