**Core Concept**
The question is testing the understanding of **peptide structure**, specifically the definition of a dipeptide. A dipeptide is a molecule consisting of two **amino acids** linked by a **peptide bond**.

**Why the Correct Answer is Right**
A dipeptide is formed when two amino acids undergo **condensation reaction**, resulting in the formation of a peptide bond between the **carboxyl group** of one amino acid and the **amino group** of another. This reaction is catalyzed by **peptidases** or **proteases**. The correct answer is a molecule that consists of two amino acids linked by a peptide bond.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a dipeptide as it does not consist of two amino acids linked by a peptide bond.
**Option B:** This option is also incorrect as it does not meet the criteria for a dipeptide.
**Option C:** This is not a dipeptide, lacking the necessary peptide bond between two amino acids.

**Clinical Pearl / High-Yield Fact**
Remember that dipeptides play a crucial role in **protein synthesis** and **metabolism**. They can also have **pharmacological effects**, such as **angiotensin-converting enzyme (ACE) inhibitors**.

**Correct Answer:** D. Carnosine