**Core Concept**
The shortest peptide refers to a type of oligopeptide or polypeptide that consists of a minimal number of amino acid residues. This concept is essential in biochemistry, particularly in understanding protein structure and function.

**Why the Correct Answer is Right**
The correct answer is a dipeptide, which is the shortest peptide that still retains the characteristic properties of a peptide. A dipeptide consists of two amino acid residues linked by a peptide bond. This bond is formed through a condensation reaction between the carboxyl group of one amino acid and the amino group of another. The resulting dipeptide has a free amino group at one end and a free carboxyl group at the other, making it a basic building block of proteins.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a monomer is not a peptide, but rather a single unit of a polymer. While a monomer can be an amino acid, it is not a peptide itself.
* **Option B:** This option is incorrect because a tripeptide consists of three amino acid residues, making it longer than a dipeptide.
* **Option C:** This option is incorrect because a protein is a large, complex molecule composed of multiple polypeptide chains. While proteins are made up of peptides, a protein is not a peptide itself.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that peptides are formed through peptide bonds, which are covalent bonds between amino acids. Understanding the structure and properties of peptides is crucial in biochemistry, particularly in the context of protein synthesis and degradation.

**Correct Answer:** A. Dipeptide.