## Core Concept
The BCG (Bacillus Calmette-Guérin) vaccine is a live, attenuated vaccine used primarily against tuberculosis (TB). It is crucial to understand the composition and preparation of the BCG vaccine for its administration. The vaccine is diluted with a specific solution to ensure its viability and efficacy.

## Why the Correct Answer is Right
The BCG vaccine is diluted with normal saline. This is because normal saline provides an isotonic environment that helps maintain the viability of the live, attenuated bacteria in the vaccine. The isotonic solution does not cause the bacterial cells to swell or shrink due to osmotic imbalances, which could lead to cell death.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a commonly used diluent for the BCG vaccine.
- **Option B:** This option is incorrect as it does not accurately represent the standard diluent used for BCG vaccine preparation.
- **Option C:** This option might seem plausible but is incorrect because the specific diluent used for BCG vaccine is not listed here.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the BCG vaccine must be protected from light and should be used within a certain time frame after reconstitution. The diluent used is typically normal saline, and the reconstituted vaccine should be stored properly to maintain its potency.

## Correct Answer Line
**Correct Answer: D. Normal saline.**