A box sits stationary on a ramp
Static friction is a force which keeps an object at rest as it is in the case of the box. It has to be overcome for the object to be set into motion.
Static force of friction is calculated as follows:
F is static force of friction.
μ is the coefficient of static friction.
η is the normal force.
Option (a) is the correct answer.
Static friction is defined as the friction that occurs between a stationary object and the surface on which it is resting.
For example, a box resting on a ramp is stationary and a static friction occurs between the box and the ramp.
Whereas when an object moves from one place to another then there exists kinetic friction between the moving object and the surface on which it moves.
Therefore, we can conclude that out of the given examples, a box sits stationary on a ramp illustrates static friction.
A box sits stationary on the ramp
The static friction keeps the box from sliding down the ramp