Correct Answer: 1674

Solution : The correct option is 1674 .

Single-celled life forms were discovered by Antonie van Leeuwenhoek in the late 17th century. Specifically, he made this discovery in 1674. Van Leeuwenhoek was a Dutch scientist and is often considered one of the pioneers of microbiology for his groundbreaking work in observing and documenting microscopic organisms, which he called "animalcules." His observations laid the foundation for our modern understanding of the microbial world.