In case of rod to complete one full revolution the rod must reach atleast the top most point as the rod does not slack like thread.
Hence for one full revolution the increase in P.E=MgL. where M is the mass of the rod.
Therefore, P.E=ΔK.E,
MgL=12Iω2=12(ML23)ω2,
(As the moment of inertia of the rod about an axis fixed to one end is I=ML23)
⇒ω=√6gL
Now, v=rω=L√6gL=√6gL (∵r=L)
Given, L=610 m and g=10 m/s2
∴v=√6×10×610=6 m/s
Correct answer:6