The correct option is A 2 cm
To just complete the circle, at the highest point, tension is zero and the gravitational force provides the necessary centripetal force.
Hence, mv2topR=mg…….(i)
Applying conservation of energy at the top and bottom points,
12mv2bottom=mg×2R+12mv2top…….(ii)
Substituting mv2top from (i) in (ii) and solving,
vbottom=√5gR
But from given velocity at bottom=√2gh
So, √2gh=√5gR
2h=5R⇒R=2 cm