In 1674, who used a single-lens microscope to examine pond water and observed dozens of protists, which he called 'animalcules', as well as spirogyra, or green algae?

Anton Van Leeuwnhoek

The correct answer is Option (4)- Anton Van Leeuwnhoek

Anton Van Leeuwenhoek is widely recognized as the father of microbiology. In 1674, he made groundbreaking observations using a single-lens microscope, discovering and likely observing protozoa for the first time. Later, he also observed bacteria, referring to them as "very little animalcules." Van Leeuwenhoek isolated these microorganisms from various sources, including rainwater, pond water, well water, and samples from the human mouth and intestine. Notably, he not only observed them but also calculated their sizes, contributing significantly to the understanding of the microscopic world and laying the foundation for the field of microbiology.