Answer: The correct answer is C.
Explanation: Frictional force is the contact force which resists the motion of one surface on another.
Mathematically,
[tex]f=\mu N[/tex]
where,
f = frictional force
[tex]\mu[/tex] = Coefficient of friction
N = normal force acting on a body
We need to find the ratio of force of sliding friction ( which is a type of frictional force) and the normal force acting on an object. This will be equal to coefficient of friction.
[tex]\mu=\frac{f}{N}[/tex]
