electric charges and feilds

the solution is like this. if we want to   make the uncharged body negatively charged, then  we brought the uncharged  sphere near to the conducting rod, the negative charges will collect on the surface  facing to the rod, while the positive charges will collect on the surface opposite to the rod, now if we connect the  the sphere to the ground then all the positive charge is neutralized as electrons flow from earth to the sphere.

sphere is left with only negative charges, now if we remove the positively charged rod from the sphere then the negative charges will equally distributed  on the surface of the sphere....

now i hope you understood why do we connect  sphere to the ground,