**Core Concept:**
The BCG vaccine is a live attenuated strain of Mycobacterium bovis, which is used to protect against tuberculosis (TB). The vaccine is diluted to reduce its potency and ensure safety for human use, while still maintaining its immunogenic properties. The dilution helps to control the side effects and complications associated with the vaccine.

**Why the Correct Answer is Right:**
The correct answer is **D**. The BCG vaccine is diluted with a sterile saline solution (sodium chloride, NaCl) to adjust its potency and reduce its potential adverse effects. Diluting the vaccine with saline ensures that the vaccine maintains its immunogenic properties but reduces its antigenicity, preventing severe reactions in recipients. Saline is used as a universal solvent and is biologically inert, ensuring the vaccine remains stable and effective.

**Why Each Wrong Option is Incorrect:**
A. Distilled water: This is not a suitable diluent for the BCG vaccine as it can denature proteins and compromise the vaccine's stability and effectiveness.
B. Alcohol: Alcohol is a denaturing agent that can damage the BCG vaccine's antigenicity, rendering it ineffective.
C. Human serum: Using human serum as a diluent might introduce contaminants and antibodies from the serum, which can compromise the vaccine's potency and safety.

**Clinical Pearl / High-Yield Fact:**
BCG vaccines prepared with different diluents may exhibit variations in immunogenicity and safety profiles. Therefore, it is crucial for healthcare professionals to be aware of the diluent used when administering the vaccine to ensure optimal efficacy and minimize adverse reactions.

**Correct Answer:** D. Sodium Chloride (Saline)