Answer:
Virus
Explanation:
The study for the pathogen of the tobacco mosaic disease started in 1886 by Adolf Mayer who thought that the pathogen is a small bacteria. Later studies on the Tobacco mosaic disease by other scientists also lead to the same observation.
Beijerinck confirmed that the tobacco mosaic causing disease is not caused by a bacteria but a soluble molecule which proliferates or replicates only when it remains present in the protoplasm of the host cell.
This infectious agent can pass through the filter of 0.1μm pore size that is smaller in size than a bacteria and the can be crystallised. This infectious was called the virus.
Thus, the Virus is the correct answer.
