Which of the following is classified as a contact force?

Frictional force is classified as a contact force because it arises from the interaction between surfaces that are in direct contact with each other. When two surfaces slide against one another, the microscopic roughness of those surfaces creates resistance to motion, which is what we experience as friction. This type of force is dependent on the normal force pressing the two surfaces together and varies with the materials in contact.

In contrast, the other forces listed—gravity, magnetic, and electrical—are all considered non-contact forces as they can act over a distance without physical contact between the objects involved. Gravity pulls objects toward each other even when they are separated by space. Magnetic and electrical forces can exert influence over distances as well, activating when objects are brought near to each other but not requiring physical contact to exert their effects.