To demonstrate the phenomenon of interference, we require two sources, which emit radiation of

the same frequency and having definite phase relationship

The correct answer is Option (4) → the same frequency and having definite phase relationship

Interference occurs when two or more waves superpose to form a resultant wave. For stable and observable interference patterns, the following conditions must be satisfied:

1. The sources must emit waves of the same frequency.

2. The sources must have a definite phase relationship (coherent sources).

If the sources have different frequencies or random phases, the interference pattern will constantly change and will not be observable.

Answer: the same frequency and having definite phase relationship