When two waves are superimposed, the result is . a) interference b) diffraction c) refraction

Interference

The Principal of superposition of the wave  

Let’s look at an example to understand superposition principal, When we drop a pin in a tank we see some circular waves when another pink is dropped we see some more waves. These waves are traveling in the same tank and at one point these waves superimpose on each other

Wave theory of light includes the phenomena of diffraction and interference.

We know that two or more wave motions travel at the same time in space. Sometimes these two wave motions combine and some physical effect takes place and interference is one effect  .When two or more waves cross each other in the same medium they both interfere which is known as the interference of waves.

there are no contact forces mentioned on the list of choices that you've provided.