Question 4
A heavy cupboard is hardest to push at the very first instant; once it is sliding it is easier to keep going. Why?
The right answer is the one marked above.
Limiting friction must be overcome to start it, and dynamic friction is smaller. The hardest push is the one that gets past the limiting value; after that you are only working against the smaller dynamic friction.
Friction certainly does not stop acting — let go and the cupboard slows and stops, which needs a force acting against its motion.