Final answer:
Friction is the force responsible for decreasing the block's mechanical energy by converting its kinetic energy into heat, acting in the opposite direction of its motion.
Step-by-step explanation:
The force responsible for the decrease in the mechanical energy of the block as it slides on a horizontal surface is friction. Friction is an external force that always acts opposite to the direction of motion, converting kinetic energy into heat as the two surfaces interact, and therefore, decreases the mechanical energy of the block. The gravitational force and normal force are balanced and therefore do not contribute to the energy decrease, whereas tension is not a factor in this context.