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I) The potential energy is given by mgh. It will be maximum when the height will be maximum i.e. at the top point of projectile or at time T/2 where T is the time of flight.
ii)The kinetic energy is maximum when the velocity is maximum. So it will be at the starting of projectile(t=0) and the end of projectile(t=T).
iii)Total mechanical energy does not change. It will always be the same.(neglecting energy loss due to air resistance)