DNA fragments separated on an agarose gel can be observed after staining with
Acetone
Ethidium bromide
Bromophenol blue
Phenolphthalein
As discussed in Gel Electrophoresis -
The separated DNA fragments can be visualized only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation after which bright orange coloured bands of DNA in an ethidium bromide-stained gel exposed to UV light will be seen. Hence, the correct option is (b).