In gel electrophoresis the separated DNA fragments can be visualised:

The correct answer is Option (4) → In UV radiation after staining with ethidium bromide

Option (1) In visible light — Incorrect. Pure DNA fragments cannot be seen in visible light.

Option (2) In visible light with staining — Incorrect. Ethidium bromide makes DNA glow only in UV light.

Option (3) In UV radiation without staining — Incorrect. Without the stain the DNA does not glow, so nothing can be seen even in ultraviolet (UV) radiation. NCERT says pure DNA fragments cannot be seen without staining.

Option (4) In UV radiation after staining with ethidium bromide — Correct. DNA fragments in the gel cannot be seen directly. They are stained with ethidium bromide and then exposed to ultraviolet (UV) radiation, when they show as bright orange coloured bands.

NCERT Extract:
Restriction Enzymes – “… The separated DNA fragments can be visualised only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation (you cannot see pure DNA fragments in the visible light and without staining). You can see bright orange coloured bands of DNA in a ethidium bromide stained gel exposed to UV light (Figure 9.3).”