What makes this question tricky is that it is not a choice between two prepositions, but between the use of study as an infinitive and as a regular noun.
When study is a verb, the thing being studied is its direct object.
Odonatologists study dragonflies and other damselflies.
When study is a noun, of indicates either the phenomenon being researched or the subjects of the research, both when study refers to a particular research project and when it refers to an entire discipline.
A study of estuary birds found high levels of selenium in their bloodstream.
There have been two studies of water pollution in the estuary, one of birds and one of damselflies.
Odonatology is the study of dragonflies and other damselflies.
The person or institution conducting or sponsoring the study would be indicated with a by prepositional phrase.
A study by the university detected selenium leaching into the water near the mine.
While medical students might be an interesting subject for psychologists or economists to study, it is more likely that the plant materials mentioned are to be used by medical students themselves. Thus one would choose option A.