**Core Concept**
Pasteurization is a process of heat treatment used to kill pathogenic microorganisms in milk, making it safe for consumption. The effectiveness of pasteurization depends on the temperature and duration of the heat treatment applied.

**Why the Correct Answer is Right**
The most commonly used method to test pasteurization is the **Bacillus stearothermophilus (Geobacillus stearothermophilus) spore test**. This thermoresistant bacterium is used as an indicator organism because its spores are highly resistant to heat and can survive pasteurization if it is not done correctly. The spores are added to the milk before pasteurization, and if the milk is properly pasteurized, the spores should be killed, whereas if they survive, it indicates inadequate pasteurization.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified and does not provide any information, making it an unclear choice.
* **Option B:** Although other methods like the **Milk Standard Plate Count (MSPC)** may be used to test the quality of milk, it is not the most commonly used method to test pasteurization specifically.
* **Option C:** This option is also not specified and does not provide any information, making it an unclear choice.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that pasteurization is not a 100% guarantee of safety, and proper handling and storage of milk after pasteurization are also crucial to prevent contamination.

**Correct Answer: D. Bacillus stearothermophilus (Geobacillus stearothermophilus) spore test.**