What is a contact force?

A contact force is defined as a force that requires physical contact between two objects to initiate or influence motion. In the case of option A, the description aligns perfectly with this definition, as contact forces include actions like friction, tension, and normal force, which are only exerted when two objects are in direct contact with each other.

In contrast, the other choices present different concepts: option B refers to non-contact forces, such as gravitational or electromagnetic forces, which can act over a distance without direct physical interaction. Option C speaks specifically to magnetic forces, which can also act at a distance and are not limited to contact interactions. Finally, option D suggests a specific behavior of forces that doesn't accurately represent contact forces because contact forces can either pull or push objects together rather than always pulling them apart. Thus, the most accurate characterization of contact force is found in option A.