Which of the following is a constant force?

(a) electrical force
(b) frictional force
(c) gravitational force
(d) magnetic force

Frictional Force

Explanation:

Electric force is depending on the charge and the distance between two charges.

Friction force depending on the coefficient of friction and the reaction which is acting on a body by surface. If we assume that the surface is same then coefficient of friction is constant and reaction is also constant.

Gravitational force is depending on the mass and the distance between two objects. There for it's changed

Magnetic force is changed due to magnetic flux and the charge and the velocity of the body.

then the more convenient answer is Friction force

no. dot product   =0 when vecs at right angles