What did van Leeuwenhoek observe that is noted in his legacy?

• A. Single-cell organisms
• B. Planets in orbit
• C. Mitochondria
• D. DNA molecules

Answer: (A)

Seeing life at the microscopic level was the turning point here. van Leeuwenhoek built tiny, powerful lenses and used them to look at pond water, dental plaque, and other samples, where he found living single-celled organisms—what he called “animalcules.” This observation is the cornerstone of his legacy because it proved that organisms exist beyond what the naked eye can detect and laid the foundation for microbiology.

The other options aren't aligned with his work: planning to orbit is astronomy, not his microscope-based discoveries; mitochondria and DNA were identified much later with more advanced biology and molecular techniques, not by van Leeuwenhoek.