Which one of the following is not a contact force ?

Which one of the following is not a contact force ?

Push force
Gravitational force
Frictional force
Strain force
This question was previously asked in
UPSC NDA-2 – 2016
The correct answer is B. A contact force is a force that acts only when there is direct physical contact between two interacting objects.
Gravitational force is a non-contact force (also known as a field force). It acts between any two objects possessing mass, even when they are separated by a distance, without any physical contact between them (e.g., the force between the Earth and the Sun, or the force between the Earth and an object in free fall). Push force, frictional force, and strain force (which arises from interactions between atoms/molecules within a material under deformation) are all examples of contact forces.
Other common examples of non-contact forces include electrostatic force (between charges) and magnetic force (between magnets or currents). The concept of forces acting at a distance was initially puzzling but is now understood in terms of fields (gravitational field, electric field, magnetic field) which mediate the interaction.