Explanation:
- T.O. Diener (Theodor Otto Diener) discovered viroids in 1971.
- Viroids are the smallest known infectious agents and consist of naked, circular, single-stranded RNA molecules. They lack a protein coat, distinguishing them from viruses.
- This RNA is free and infectious, capable of causing diseases in plants (e.g., potato spindle tuber disease).
Other Options:
1. Bacteriophage: These are viruses that infect bacteria. They were discovered earlier by scientists like Twort and d'Herelle, not by Diener.
2. Free infectious DNA: Not applicable here, as viroids consist of RNA, not DNA.
3. Infectious protein: Refers to prions, which are infectious protein particles discovered by Stanley
B. Prusiner in 1982.
Thus, T.O. Diener is credited with discovering free infectious RNA, which corresponds to viroids.