Potato spindle-tuber disease, which causes potatoes to become gnarled and cracked, is caused by an infectious agent consisting of a small circular RNA molecule that totally lacks a protein coat. These infectious agents are thought to exert their effects by interfering with the cell's normal path of gene expression. Such an infectious agent is known as a(n):.
a. provirus
b. bacteriophage
c. prion d. viroid
e. girus

d. viriod

Potato spindle-tuber disease, which causes potatoes to become gnarled and cracked, is caused by an infectious agent consisting of a small circular RNA molecule that totally lacks a protein coat. These infectious agents are thought to exert their effects by interfering with the cell's normal path of gene expression. Such an infectious agent is known as a(n): viriod.

Explanation:

Viriods are very small , covalently closed, circular RNA molecules capable of autonomous replication and induction of disease.

They range in size from approximately 20nm and are infectious particles.

They consist only of Nucleic acid (RNA) and exert their effects by interfering with the cell's normal path of gene expression.

They mainly cause diseases in Plants.

As in the potato spindle-tuber disease, the infectious agent has all the characteristics as that of a virioid. Hence Viriod is the right choice.