Correct option is (A)
To complete the circle, \(\frac{mv_0^2}{R}=mg\Rightarrow v_0^2=gR\)
Now, from conservation of energy: K1+U1=K2+U2
\(\frac{1}{2}mv^2+0=\frac{1}{2}mv_0^2+mg(2R)\)
\(\Rightarrow v^2=gR+4gR\Rightarrow v=\sqrt{5gR}\)
\(\Rightarrow v=\sqrt{5\times 10\times 6.4}=17.7m/s\)