Antonie von Leeuwenhoek was the first scientist to identify:

Antonie von Leeuwenhoek is famously recognized as the first scientist to observe and describe bacteria. His advancements in microscopy allowed him to explore the microscopic world in detail, leading to his discovery of tiny living organisms, which he referred to as "animalcules." His observations were significant because they initiated the field of microbiology and challenged the prevailing notions of life at that time. This pioneering work laid the groundwork for understanding the role of microorganisms in both health and disease.

The other options pertain to entities or structures that were either unknown or not classified in the way we understand them today during Leeuwenhoek's time. While he did observe various forms of cells and other microorganisms, his most notable contribution was identifying bacteria. Therefore, this answer accurately reflects his significant role in the field of biology.