Unicellular microscopic organisms were first studied by-
a. Robert Hooke
b. Priestley
c. Pasteur
d. Leeuwenhoek

Hint: Microscopic organisms are the organisms which are too small to be seen by the naked eyes and thus can only be observed with the aid of a microscope. And so, it is a matter of great importance to know as to how the first unicellular microscopic organisms were first studied and who studied them.

Complete answer:
Let us first know the contribution of various eminent scientists given in the options-
> Robert Hooke- Also called the “England’s Leonardo”, Robert Hooke was an English scientist and architect. He was the first who visualised a microorganism using a microscope. He is also known for the coining of the term ‘cell’.

> Priestley- Joseph Priestley was an English chemist, popularly known for the discovery of oxygen and nine other gases. He is also known for the discovery of the carbon cycle.

> Pasteur- A French microbiologist, biologist, and chemist, Louis Pasteur is well renowned for discovering the principles of pasteurization, vaccination and microbial fermentation. He is also known for his remarkable breakthroughs in the causes and prevention of diseases. He was also responsible for disproving the doctrine of spontaneous generation and provided direct support for the germ theory of disease.

> Leeuwenhoek- Commonly known as “the Father of Microbiology”, Antonie Van Leeuwenhoek was a Dutch businessman and scientist. He designed his own single-lensed microscope and by using this microscopic design, he for the very first time experimented microbes. These microbes were initially referred to as ‘dierkens’ by Leeuwenhoek, meaning “small animals” in Dutch. They were called ‘animalcules’, when translated into English. He was the first to relatively determine their size through his experiments. These ‘animalcules’ are now mostly referred to as unicellular organisms.

Hence, the correct answer is option (D).

Note: Microscopic observations of muscle fibers, red blood cells,spermatozoa, bacteria, and blood flow in capillaries was also first documented by Antonie van Leeuwenhoek. It is noteworthy to mention that Antonie van Leeuwenhoek did not write any book and his discoveries came to light only after the Royal Society published his letters.