Viroids are discovered by:

Correct option is A

The correct answer is (A) Diener

Explanation:
• T.O. Diener (Theodor Otto Diener), a plant pathologist at the USDA, discovered viroids in 1971.
• He was investigating the cause of Potato Spindle Tuber disease and found it was caused by an agent much smaller than a virus.
• Viroids are the smallest known infectious agents, consisting solely of a short strand of circular, single-stranded RNA without a protein coat (capsid).

Information Booster:
• Unlike viruses, viroids do not code for any proteins.
• They replicate within the host cell's nucleus or chloroplasts using host enzymes.

Additional Knowledge:
• Prusiner (Option B): Stanley Prusiner discovered Prions (proteinaceous infectious particles) which cause diseases like Mad Cow Disease.
• Doi (Option C): Y. Doi and colleagues (1967) discovered Phytoplasmas (formerly MLOs) as the cause of mulberry dwarf disease.
• Kassanis (Option D): B. Kassanis is well known for his work on satellite viruses.