Identify the part labeled as $'D'$ in the given figure.

$T. O.$ Diener discovered a new infectious agent that was smaller than viruses and has the following characteristics:
$(i)$ It causes potato spindle tuber disease.
$(ii)$ It has free $RNA$.
$(iii)$ The molecular weight of $RNA$ is low.
Identify the infectious agent.

What are prions?

Which of the following is an example of a lichen?

Who discovered viroids?

Which of the following is an incorrect statement regarding viruses?