**Core Concept**
Antonie van Leeuwenhoek is considered the "Father of Microbiology" due to his pioneering work in discovering microorganisms using a handcrafted microscope. His observations of microorganisms revolutionized the field of microbiology and laid the foundation for modern microbiological research.

**Why the Correct Answer is Right**
Van Leeuwenhoek's discovery of microorganisms, including bacteria, yeast, and protists, was a groundbreaking finding that challenged the prevailing understanding of the natural world. His observations were made possible by his innovative microscope design, which allowed for the visualization of microorganisms that were previously invisible to the naked eye. This discovery marked the beginning of microbiology as a distinct scientific discipline.

**Why Each Wrong Option is Incorrect**
**Option A:** Louis Pasteur is credited with the germ theory of disease, but he did not discover microorganisms.
**Option B:** Robert Koch made significant contributions to the field of microbiology, but he did not discover microorganisms.
**Option C:** Gregor Mendel is considered the "Father of Genetics," but he did not make significant contributions to microbiology.

**Clinical Pearl / High-Yield Fact**
Van Leeuwenhoek's discovery of microorganisms highlighted the importance of microbiological research in understanding disease and promoting public health.

**Correct Answer:** D. The discovery of microorganisms by Antonie van Leeuwenhoek.