The correct answer is $\boxed{\text{B) 18° 48′}}$.
The angle of friction is the angle between the applied force and the normal force, when the block is just on the point of motion. The normal force is the force that the surface exerts on the block, perpendicular to the surface. The applied force is the force that we apply to the block, in the direction of motion.
The angle of friction can be calculated using the following formula:
$$\mu = \tan \theta$$
where $\mu$ is the coefficient of friction and $\theta$ is the angle of friction.
The coefficient of friction is a constant that depends on the materials of the block and the surface. It can be found in tables or determined experimentally.
In this case, the weight of the block is 50 kg. The normal force is equal to the weight of the block, which is 500 N. The applied force is 18 kg, which is 180 N.
The coefficient of friction is given by:
$$\mu = \frac{180}{500} = 0.36$$
The angle of friction is given by:
$$\theta = \arctan \mu = 18° 48’$$