Part a)
As we know that energy stored inside the capacitor is given as
for a given capacitor we know
Now we can use it in above equation to find the energy
PART b)
If two negative charges are hold near to each other and then released
Then due to mutual repulsion they start moving away from each other
Due to mutual repulsion as the two charges moving away the electrostatic potential energy of two charges will convert into kinetic energy of the two charges.
So here as they move apart kinetic energy will increase while potential energy will decrease
Part c)
As we know that capacitance is given as
here we know that