Correct Answer - a
Torque due to magnetic field `tau_(mag)=MB_0=ipiR^2B_0....(i)`
Torque due to weight about the point where string is connected
`tau_("weight")=mgR....(ii)`
If ring remains horizontal, then `tau_(mag)=tau_("weight")`
`IpiR^2B_0=mgRimpliesI=(mg)/(piRB_0)`.