The correct option is C minimizes the electric field around it.
The whole purpose of the generator is to retain charge and create a region of a high potential. To prevent leakage of charge through corona discharge or other such phenomena, we have to avoid using spikey or narrow conductors.
So, rounded terminal is used in order to minimize the electric field around it, allowing greater potentials to be achieved without ionisation of the air, or other dielectric gas, surrounding.
Hence, option (b) is the correct alternative.