Final answer:
The red block being discussed is subjected to a combination of gravitational force, normal force, and frictional force while sliding on a horizontal surface. Therefore all give option is correct.
Step-by-step explanation:
When a red block is sliding on a horizontal surface, there are typically three types of forces acting upon it: gravitational force, normal force, and frictional force.
The gravitational force pulls the block towards the Earth, the normal force acts perpendicular to the surface supporting the block (due to electromagnetic repulsion), and the frictional force opposes the direction of the block's motion. The combination of forces determines the net acceleration and movement of the block.
According to Newton's First Law of Motion, which addresses inertia, friction is considered an external force because it acts upon the object from outside, not originating from within the object itself.
Furthermore, when evaluating scenarios where only gravity acts upon an object, such as a car moving over the top of a hill at a certain speed, only the gravitational force would be considered.
However, in the case of a block on a frictionless surface, only the forces acting perpendicular and parallel to the surface should be considered.
Therefore all give option is correct.