Reye's syndrome is diagnosed by using _______ stain -
Read following 2 facts :-

"Oil red O stain is used to stain fat".
"Characteristic histopathologic change in the cells in Rey's syndrome is fat accumulation. Glycogen is completely absent from the cells".

These changes are seen in the cells of liver, brain & kidneys. So after combining these 2 facts, you can easily get the answer.
Now something more about stains
Reticulin stain
Oil-red-O stain
PAS stain
Mustein stain