(a) Since our body and the surface of earth, both are conducting, therefore, our body and the ground form an equipotential surface. As we step out into the open from our house , the orignal equipotential surfaces of open air charge, keeping our body and the ground at the same potential. That is why we donot get an electric shock.
(b) Yes, the man will get the shock. This is because the steady discharging current of the atmostphere charegs up the aluminium sheet gradually adn raises its voltage to an extent depending on the capacitance of the condenser formed by the aluminium sheet, the ground and the insulating slab.
(c) The atomosphere is being charged continously be thunderstoms and lighting all over the globe. It is also discharging due to the small conductivity of air. The two opposing process, on an average,are in equilibrium. Therefore the atmosphere keeps charged.
(d) During a lighting the electrical energy of the atmosphere is dissipated in the form of light heat and sound.