Which nucleic acid is present in viruses?

Host specificity is a:

What acts as the phycobiont in the organism shown in the given figure?

$A$ scientist $X$ demonstrated that the extract of infected plants of $Y$ could cause infection in healthy plants and called the fluid as "Contagium vivum fluidum". Identify $X$ and $Y$.

Match the following columns.

Column $I$	Column $II$
$(A)$ $DJ$ Ivanowsky	$(i)$ Viroids
$(B)$ $MW$ Beijerinck	$(ii)$ Crystallised virus
$(C)$ $WM$ Stanley	$(iii)$ Contagium vivum fluidum
$(D)$ $TO$ Diener	$(iv)$ Mosaic disease of tobacco

Which of the following organisms acts as an air quality indicator?