I am going to guess:
b. nucleic acids.
The deal here is that a ribosome is partly made of RNA (rRNA), and when active, it is bound to mRNA.
The problem is that you're going to take several trials to determine how much stain to use and how long to "destain." The cytosol has plenty of RNA that isn't bound in or to ribosomes.
this is the correct answer for If you wanted to stain the ribosomes in a cell, you could use a dye that stains?