Bacteria were first discovered bya. Robert Koch b. L Pasteurc. Robert Hooked. A.V Leeuwenhoek

The correct answer is d. A. V. Leeuwenhoek. Antonie van Leeuwenhoek, a Dutch scientist, is credited with the discovery of bacteria. In the 17th century, using his self-made microscopes, Leeuwenhoek observed and described various microorganisms, including bacteria, for the first time. His discoveries laid the foundation for the field of microbiology.

The other options mentioned in your question are notable figures in the history of science, but they are associated with different achievements:

a. Robert Koch: He is known for his work in the field of medical microbiology and is considered one of the founders of modern bacteriology. Koch developed techniques to isolate and identify specific bacteria responsible for causing diseases such as tuberculosis and cholera.

b. L. Pasteur: Louis Pasteur was a French chemist and microbiologist. He made significant contributions to the understanding of infectious diseases, fermentation, and pasteurization. Pasteur is famous for his experiments that disproved the theory of spontaneous generation and supported the germ theory of disease.

c. Robert Hooke: Hooke was an English scientist who made important contributions to various scientific disciplines, including biology and microscopy. He is known for his book "Micrographia," published in 1665, which introduced the term "cell" and described the cellular structure of cork. While Hooke observed microorganisms, he did not discover bacteria specifically.