• Discovery of Viroids: In 1971, the Swiss-American plant pathologist Theodor O. Diener (T. O. Diener) discovered that the causal agent of Potato Spindle Tuber Disease (PSTD) was not a virus, but a novel subviral infectious entity.
• Nature of Viroids: Diener proved that the agent consisted exclusively of a tiny, circular, single-stranded non-protein-coding RNA molecule ($246-467\text{ nt}$) devoid of any protective protein coat, for which he coined the term 'viroid'.
• Other Scientists Listed:
• W. M. Stanley (1935): Crystallized Tobacco Mosaic Virus (TMV).
• F. C. Bawden & N. W. Pirie (1937): Demonstrated that TMV virions contain both protein and RNA.
• Conclusion: T. O. Diener reported that potato spindle tuber disease is caused by viroids (Option 4).

Correct option: (4) T.0. Diener

• Buffers for Plant Virus Extraction and Maintenance:
• Role of Buffer: Plant sap is naturally acidic and rich in oxidizing polyphenols that rapidly precipitate or denature viral nucleocapsids during tissue disruption.
• Phosphate Buffer: A neutral phosphate buffer ($0.05-0.1\text{ M}$ potassium or sodium phosphate buffer, $pH\ 7.0-7.2$), often supplemented with reducing agents (ascorbic acid, sodium sulfite), provides superior buffering capacity at physiological $pH$, maintaining virion ionic stability, solubility, and high infectivity during mechanical isolation and transmission.
• Conclusion: Phosphate buffer is best suited (Option 2).

Correct option: (2) Phosphate buffer

• Ultracentrifugation in Plant Virus Purification:
• Statement (I) is true: Differential high-speed sedimentation (typically $80,000-120,000\times g$ in a preparative ultracentrifuge) pellets dense virions, significantly improving virus recovery and yield from crude clarification extracts.
• Statement (II) is true: High-speed pelleting allows viral nucleocapsids to sediment to the tube bottom while low-molecular-weight host contaminants, soluble proteins, ribulose bisphosphate carboxylase (RuBisCO), and polyphenols remain in the supernatant, achieving simultaneous purification and concentrated pellet recovery.
• Conclusion: Both Statement (I) and Statement (II) are true (Option 1).

Correct option: (1) Both Statement (I) and Statement (II) are true.