B in the sliding process, kinetic energy decreases, gravitational potential energy decreases, electric potential energy increases and elastic potential energy increases. Because the increase of electric potential energy is equal to the decrease of gravitational potential energy, according to the conservation of energy, the increase of elastic potential energy is equal to the decrease of kinetic energy of slider. When the slider reaches the lowest point, the work done by overcoming the spring force is equal to the increase of elastic potential energy, that is, the decrease of kinetic energy, which is 12mv2, so b is correct.
C. Work done by electric field force is equal to the increase of electric potential energy, and work done by gravity is equal to the decrease of gravitational potential energy. Because the increase of electric potential energy of slider is equal to the decrease of gravitational potential energy of slider, the algebraic sum of electric field force and gravitational work is zero, while the change of kinetic energy of slider is mv2. According to the theorem of kinetic energy, the algebraic sum of electric field force, gravity and spring elasticity is equal to the change of kinetic energy of slider. So, C is wrong.
D kinetic energy, gravitational potential energy and elastic potential energy are collectively referred to as the mechanical energy of the system. According to the law of conservation of energy, when the potential energy increases, the mechanical energy of the system decreases. When the slider moves to the lowest point, the acceleration is the largest, and the electric field force does the most negative work, that is, the electric potential energy increases the most and the mechanical energy of the system is the smallest. So, d is wrong.
So: ab.