Final answer:
Anton van Leeuwenhoek is recognized for developing the first single-lens microscope and was the first to observe microorganisms, also known as 'animalcules', such as bacteria and protozoa. His work laid the foundation for microbiology, and his observations were vital in understanding the microscopic world.
Step-by-step explanation:
The correct answer to the question of who developed the first single-lens microscope is B. Anton van Leeuwenhoek. Known as the 'Father of Microbiology', Leeuwenhoek was a Dutch tradesman and scientist who crafted high-quality single-lens microscopes. He used these microscopes to make groundbreaking observations, notably being the first to observe 'animalcules', or microorganisms, which included bacteria and protozoa. His discoveries marked significant milestones in scientific history, contributing to the birth of microbiology.
Answering the bonus question provided, the first to observe 'animalcules' under the microscope was indeed A. Antonie van Leeuwenhoek. His meticulous work in lens making resulted in microscopes capable of powerful magnification, enabling him to be the inaugural person to document human cells and bacteria to the scientific community through his letters to the Royal Society of London beginning in 1674.