Hello Aspirant.
Since it's a frictionless surface, the total potential energy of the body will be converted to kinetic energy.
So, mgh=½mv^2
h=v^2/2g
Since the minimum velocity at the lowest point to complete the loop is √(5gr)
So, h= (5gD/2)/2g
Therefore the minimum height= 5/4D
Hope it helps.
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