Final answer:
It was unsurprising that Antonie van Leeuwenhoek detected bacteria and protozoa because he had a microscope capable of detecting them. The correct answer is option (a).
Step-by-step explanation:
It was unsurprising that van Leeuwenhoek detected bacteria and protozoa because he had a microscope capable of detecting them. Antonie van Leeuwenhoek, often called the Father of Microbiology, was not formally trained as a scientist but was a skilled lens maker. He deliberately improved upon the existing designs of microscopes in the 17th century. His development of powerful single-lens microscopes enabled him to observe the previously invisible world of microorganisms. With his self-made lenses, van Leeuwenhoek reported on his discovery of "animalcules", which we now understand as bacteria and protozoa, by examining a drop of water, plaque scraped from his teeth, and other specimens.
While van Leeuwenhoek was a pioneer in the field, he was not an expert in microbiology as the field did not exist yet. He was also not specifically looking for bacteria and protozoa, but his curiosity and his powerful microscopes led him to observe them. His discoveries were largely due to his exceptional skills in crafting lenses that provided better resolution and magnification than other microscopes of his time, which were more akin to modern light microscopes in their strength.
Therefore, the correct option in final answer to the question 'Why was it unsurprising that van Leeuwenhoek detected bacteria and protozoa?' is (a) He had a microscope capable of detecting them.