Final answer:
A botanist can confirm disease in a tomato plant is caused by a viroid by identifying the presence of circular RNA without a protein coat, which distinguishes viroids from viruses.
Correct option is d) By observing the presence of circular RNA without a protein coat.
Step-by-step explanation:
To confirm that the agent causing disease in a tomato plant is a viroid and not a virus, a botanist could look for the presence of circular RNA without a protein coat, which is a characteristic of viroids. Unlike viruses, viroids consist only of a short strand of circular RNA capable of self-replication and they do not have a protein coat or capsid.
Viroids are small infectious agents that consist of a short, circular RNA molecule without a protein coat. Unlike viruses, viroids lack the protein coat (capsid) that typically surrounds the genetic material in viruses. Viroids are known to cause diseases in plants.
So, if the botanist observes the presence of a circular RNA molecule without a protein coat in the pathogen affecting the tomato plant, it would be indicative of a viroid rather than a virus. Option d, "By observing the presence of circular RNA without a protein coat," is the correct choice.