Final answer:
Friction is an external force that opposes the motion of an object. The block slows down due to the force of friction opposing its motion.
Step-by-step explanation:
The block slows down due to the force of friction opposing its motion. Friction is an external force that acts opposite to the direction of motion. When a block slides across a horizontal floor, the friction force between the block and the floor opposes the motion of the block, causing it to slow down.