Final answer:
The frictional force is responsible for the motion of a 2 kg block sliding along a horizontal tabletop, not the gravitational or normal forces, and no tension force is involved unless there's a rope or cable.
Step-by-step explanation:
The force responsible for the motion of a block of mass 2 kg sliding along a horizontal tabletop is not the gravitational force, which acts vertically downwards, nor the normal force, which acts perpendicular to the surface to support the object's weight. The tension force is not involved unless there is a rope or cable pulling the block. Therefore, the responsible force is the frictional force, which is a resistance force that acts opposite to the direction of motion. When the block slides on the tabletop, the frictional force can either be the force that slows it down (if it's already in motion) or the force exerted by another object (like a hand or a pushing device) that is moving the block via friction.