ANSWER
a. interference
Step-by-step explanation
The described setup is known as Young's experiment. When a single laser beam passes through two small slits in a slide, the beam splits and they take different directions,
Two waves get to the screen, one from each slit. Since they are separated, these waves become out-of-phase when they get to the screen,
This causes interference.
If one wave hits the screen with a crest and the other does so with a trough, they will cancel out creating a dark area. This is called destructive interference.
If both waves hit the wall with a crest, they will add up, creating a bright area. This is called constructive interference.
Between one and the other, the light is dimmed until the dark area. This is because there the waves with different parts of the wave that add or subtract creating a less bright area. This is called a diffraction pattern.
Hence, the behavior that causes the described pattern is interference.