The contact force that involves two objects being in contact with each other is friction.
1. **Friction:** Friction is a contact force that occurs when two surfaces are in contact and moving relative to each other, resisting their motion. For example, when you push a book along a table, the force opposing the motion is friction. This force arises due to the interactions between the molecules of the two surfaces.
2. **Gravity:** Gravity, on the other hand, is not a contact force. It is a force of attraction between two masses, such as the Earth and an object. Gravity acts at a distance and does not require direct contact between the objects to exert its force.
Therefore, in the given options, the contact force that involves two objects being in contact is friction, as it occurs when two surfaces are in contact and one object moves relative to the other, generating the force of friction.
