A Russian scientist discovered that certain diseases could be transmitted by an agent that could pass through a very fine filter which is called:

Correct Answer: C. Viruses

The correct answer is C. Viruses.
The Russian scientist referred to is Dmitri Ivanovsky. In 1892, he conducted experiments with tobacco plants and discovered that the agent causing the tobacco mosaic disease could pass through filters fine enough to block bacteria. This marked the discovery of viruses, which are much smaller than bacteria and capable of passing through such filters.
Ivanovsky’s work laid the foundation for the study of virology, although it was later confirmed and expanded upon by others, such as Martinus Beijerinck.