**Core Concept**
The question is testing the student's knowledge of basic amino acids, which are a class of amino acids that have a basic side chain. These amino acids play a crucial role in various biological processes, including protein structure and function.

**Why the Correct Answer is Right**
The correct answer is **lysine**, which is the most basic amino acid due to the presence of an amino group (-NH2) in its side chain. This amino group has a pKa of around 10.5, making it the most basic among the standard amino acids. Lysine's basic nature allows it to participate in various biochemical reactions, such as protein-protein interactions and enzyme catalysis. The high pKa value of lysine's amino group also enables it to form salt bridges with negatively charged groups, contributing to protein stability.

**Why Each Wrong Option is Incorrect**
**Option A:** Arginine - While arginine is a basic amino acid, its pKa value is lower than that of lysine, making it less basic.
**Option B:** Histidine - Histidine's basicity is pH-dependent, and it becomes more basic at higher pH values, but lysine remains more basic across a wider pH range.
**Option C:** Ornithine - Ornithine is a non-essential amino acid, but its basicity is lower than that of lysine.

**Clinical Pearl / High-Yield Fact**
Remember that the basicity of amino acids is crucial for protein function, and lysine's unique properties make it an essential amino acid for human health.

**Correct Answer: B. Histidine**