The correct option is C 12.9 m/s
The tension in the string is highest at the lowest point during the rotation and the tension in the string is lowest at the highest point in the rotation.
Tension at the lowest point = Tmax=mv2r+mg
Tension at the highest point =
Tmin=mv2r−mg
TmaxTmin=mv2r+mgmv2r−mg=41⇒v2+rgv2−rg=41⇒4v2−4rg=v2+rg⇒v2=5rg3⇒v=√5rg3=√5×10×103=12.9 m/s