**Core Concept**
Viroids are small, single-stranded RNA molecules that infect plants and cause various diseases. They are distinct from viruses in that they do not have a protein coat or envelope, and their replication does not involve the host cell's machinery.

**Why the Correct Answer is Right**
The correct answer, **c) Have only genetic material**, is accurate because viroids consist solely of a small, circular RNA genome without any protein coat or envelope. This is in contrast to viruses, which have a protein coat and sometimes an envelope. The lack of a protein coat means that viroids cannot be visualized using a light microscope, making option d incorrect.

**Why Each Wrong Option is Incorrect**
* **Option a:** Cause tumors in animals - This is incorrect because viroids primarily infect plants and cause diseases such as potato spindle tuber viroid (PSTVd), which causes stunted growth and abnormal tuber formation.
* **Option b:** Lack envelope like covering - While this is partially true, it's not the most accurate statement. Viroids lack both a protein coat and an envelope, but the correct answer is the one that specifically highlights their genetic material.
* **Option d:** Visible on light microscope - This is incorrect because viroids are too small to be visible on a light microscope, unlike some viruses that can be seen.

**Clinical Pearl / High-Yield Fact**
Viroids are obligate parasites, meaning they require a host cell to replicate and cause disease. They have a unique replication mechanism that involves the host cell's RNA polymerase, but not its protein synthesis machinery.

**Correct Answer: C. Have only genetic material**