Leeuwenhoek was a scientist who discovered living micro organisms in a drop of water while researching, that's why the answer is B.Hope this will be helpful for you!!!!!!!!!

Leeuwenhoek observed living microorganisms in a drop of pond water. This observation was a significant discovery in the field of microbiology and helped pave the way for the understanding of the microscopic world.

1. Introduction:
- Antonie van Leeuwenhoek was a Dutch scientist who is often referred to as the "Father of Microbiology."
- He was the first person to observe living microorganisms using a simple microscope that he designed and constructed himself.

2. Pond water samples:
- Leeuwenhoek collected samples of pond water, particularly from stagnant ponds.
- He believed that these samples would provide interesting microscopic organisms to study.

3. Microscope design:
- Leeuwenhoek's microscope consisted of a single lens, which he ground and polished to a high degree of precision.
- This simple design allowed him to achieve high magnification and resolution.

4. Observations:
- When Leeuwenhoek examined the pond water samples under his microscope, he made several fascinating observations.
- He noticed the presence of tiny, moving organisms that he referred to as "animalcules."
- These animalcules were unicellular and exhibited various shapes and movement patterns.

5. Significance of the discovery:
- Leeuwenhoek's observation of living microorganisms in pond water challenged the prevailing belief at that time, which held that life only existed in larger organisms.
- This discovery provided evidence for the existence of a microscopic world teeming with life.
- It laid the foundation for the field of microbiology and the study of microorganisms.

6. Further discoveries:
- Inspired by his initial observations, Leeuwenhoek went on to examine a wide range of samples, including blood, saliva, and even his own dental plaque.
- Through his meticulous observations, he described and documented various microorganisms, including bacteria and protozoa.

In conclusion, Leeuwenhoek's observations of living microorganisms in a drop of pond water using his self-designed microscope were groundbreaking. This discovery challenged existing scientific beliefs and laid the foundation for the field of microbiology. His meticulous observations opened up a new world of microscopic life and paved the way for further advancements in the understanding of microorganisms.