To demonstrate the phenomenon of interference, required two sources of light must have

same frequency and have a constant phase difference

The correct answer is Option (3) → same frequency and have a constant phase difference

Interference is the phenomenon where two waves superpose to form a resultant wave of greater, lower, or the same amplitude.

To observe stable and sustained interference, the two light sources must be coherent, meaning:

• They should have the same frequency.
• They should maintain a constant phase difference.

If the phase difference changes randomly, the interference pattern will not be observable or stable.