T.O. Diener discovered a

Options

A. Free infectious DNA
B. Infectious protein
C. Bacteriophage
D. Free infectious RNA

Answer

**Step-by-Step Solution:**

1. **Identify the Discoverer and Context**: T.O. Diener is known for his work in plant pathology, specifically related to the potato spindle tuber disease. Understanding the context of his discovery is crucial.

2. **Understand the Nature of the Discovery**: Diener discovered a unique infectious agent that was not classified as a virus. This agent was responsible for causing the potato spindle tuber disease.

3. **Recognize the Characteristics of the Agent**: The agent discovered by Diener is characterized as being capsid-free and consists of a single strand of RNA. This distinguishes it from typical viruses, which usually have a protein coat (capsid).

4. **Define the Term**: The term used to describe this type of infectious agent is "Viroid." Viroids are small, circular RNA molecules that can infect plants and do not encode proteins.

5. **Select the Correct Option**: Based on the information provided, the correct answer to the question is "pre-infectious RNA," which refers to the nature of the viroid as it does not have a protein coat and is infectious.

6. **Conclusion**: Therefore, T.O. Diener discovered a pre-infectious RNA, which is a type of viroid.

**Final Answer**: T.O. Diener discovered a pre-infectious RNA (Viroid).

---