Correct Option: A

Van Leeuwenhoek discovered “protozoa” - the single-celled organisms and he called them “animalcules”. He also improved the microscope and laid foundation for microbiology. He is often cited as the first microbiologist to study muscle fibers, bacteria, spermatozoa and blood flow in capillaries. Although, he did not have much education or a scientific background, yet he defied all odds to be reckoned as a great scientist through his skillful observations, insight and unmatched curiosity. He revolutionized biological science by exposing microscopic life to the world.