Answer:
E. the waves have different wavelengths
Step-by-step explanation:
Light wave produced from two-slit diffracts from each slit, because the slits are narrow. These waves overlap and interfere constructively and destructively.
Again, if the paths taken by the two waves differ by any multiple of half-wavelengths (¹/₂, ³/₂, ⁵/₂ etc), then destructive interference occurs. Also, if the paths taken by the two waves differ by any whole number multiple of wavelengths (1, 2, 3, 4, etc), then constructive interference occurs.
Thus, one gets interference of the two waves on a screen in two-slit interference because the waves have different wavelengths