Final answer:
The correct answer is option d) The direction of motion will change.
Step-by-step explanation:
When Carlos starts pulling the block at an angle to the horizontal, the force can be resolved into two components: one parallel to the surface (horizontal), and one perpendicular (vertical). The vertical component of the force affects the normal force on the block. If he pulls upward at an angle, this decreases the normal force, and consequently, the friction force.
Since friction is the force that opposes the motion, reducing friction could cause the block to move with an increased speed, if the pulling force provides sufficient net force in the direction of motion. The exact effect on the block's motion depends on the magnitude of the pulling force and the angle of pull.
The frictional force always opposes the motion of the block. If the block is sliding forward, friction acts in the opposite direction to the movement. It does not depend on molecular interactions deforming the block but rather the resistance between the surfaces in contact.
The coefficient of kinetic friction between two surfaces remains constant regardless of the relative speed of the surfaces in contact with one another, so long as the nature of the surfaces remains unchanged.