The infectious agent that is smaller to viruses and caused potato spindle tuber disease is

Options

A. Virion
B. Prions
C. Virusoid
D. Viroid

Answer

To solve the question regarding the infectious agent that is smaller than viruses and causes potato spindle tuber disease, we can follow these steps:

### Step-by-Step Solution

1. **Understanding the Question**: The question asks for an infectious agent that is smaller than viruses and is responsible for potato spindle tuber disease.

2. **Evaluating the Options**:
- **Option 1: Viron**: A viron refers to a complete virus particle, which includes a protein coat (capsid) and nucleic acid (either RNA or DNA). Since a viron is a virus itself, it cannot be smaller than viruses. Therefore, this option is incorrect.

- **Option 2: Prions**: Prions are infectious agents composed solely of protein that can induce abnormal folding of normal cellular proteins, particularly in the brain. While they are indeed smaller than viruses, they do not cause potato spindle tuber disease. Hence, this option is also incorrect.

- **Option 3: Virusoid**: Virusoids are circular, single-stranded RNA molecules that depend on viruses for their replication and encapsulation. They do not cause diseases on their own. Therefore, this option is incorrect as well.

- **Option 4: Viroid**: Viroids are the smallest known infectious agents and consist of a short strand of circular RNA without a protein coat. They are known to cause diseases in plants, including potato spindle tuber disease. Thus, this option is correct.

3. **Conclusion**: After evaluating all the options, the infectious agent that is smaller than viruses and causes potato spindle tuber disease is **viroid**.

### Final Answer
The infectious agent that is smaller than viruses and causes potato spindle tuber disease is **viroid**.