Which most simplified form of the law of conservation of energy describes the motion of the block as it slides on the floor from the bottom of the ramp to the moment it stops?

Law of conservation of mechanical energy.

Explanation:

During motion of a block sliding  on the floor from the bottom of the ramp to the moment it stops , kinetic energy is converted into potential energy so that total energy remains constant . No other conversion is taking place , if we consider friction to be absent in the system . Potential energy and kinetic energy form part of mechanical energy . So we can say that mechanical energy is conserved . Hence , system is following law of conservation of mechanical energy.