Final answer:
For two waves to interfere constructively, the path length difference must be an integral multiple of the wavelength, allowing the waves to arrive in phase and reinforce each other.
Step-by-step explanation:
The necessary condition for two waves to interfere constructively is that the path length difference from two slits must be an integral multiple of the wavelength. This means that the waves must arrive at a point in phase for them to add up, or reinforce each other, resulting in a larger amplitude. The correct answer to the options given is (b) The path length difference is an even multiple of the wavelength. By comparison, it is worth noting that destructive interference occurs when the path length difference is a half-integral multiple of the wavelength because the waves would arrive out of phase, causing them to cancel each other out.