Question

Which of the four fundamental forces is mainly responsible for contact forces?

Answer 1

The electromagnetic force.

Explanation:

The electromagnetic force explains why the atoms in your body do not scatter and you sink through your chair.
In really simple terms, your body repels the chair on an atomic scale due to interactions between the atoms of both entities and the electromagnetic force is responsible for this.

Hence, if you can come into contact with another object without actually sinking through it (due to the electromagnetic force), it is thus responsible for all contact forces governed by Newton's Laws, otherwise, no contact force would exist if you cannot touch another object.