The correct answer is C. The angle of friction is the angle between the normal reaction and the resultant of normal reaction and limiting friction.
The angle of friction is a measure of the force of friction between two surfaces. It is defined as the angle between the normal reaction and the resultant of normal reaction and limiting friction. The limiting friction is the maximum value of the force of friction that can be exerted between two surfaces.
The angle of friction can be calculated using the following formula:
$\theta = \arctan \left(\frac{f_f}{N}\right)$
where $\theta$ is the angle of friction, $f_f$ is the force of friction, and $N$ is the normal reaction.
The angle of friction is important in many engineering applications, such as the design of brakes and clutches. It is also used in the determination of the coefficient of friction.
Option A is incorrect because the ratio of the friction and the normal reaction is the coefficient of friction.
Option B is incorrect because the force of friction when the body is in motion is the kinetic friction.
Option D is incorrect because the force of friction at which the body is just about to move is the limiting friction.