Correct Answer - B
Let the direction of current in wire `PQ` is from `P` to `Q` and its magnitude be `I`.
The magnetic moment of the given loop is
`M=-Iabhatk`
Torque on the loop due to magnetic force is
`tau_1=MxxB`
`=(-Iabhatk)xx(3hati+4hatk)B_0hati`
`=-3IabB_0hatj`
Torque on weight of the loop about axis `PQ` is
`tau_2=rxxF=(a/whati)xx(-mghatk)`
`=(mga)/2hatj`
We see that when the current in the wire `PQ` is from `P` to `Q, tau_1` and `tau_2` are in opposite directions, so they can cancel each other and the loop may remain in equilibrium. so, the direction of current I in wire `PQ` is from `P` to `Q`. Further for equilibrium of the loop
`|tau_1|=|tau_2|`
`or 3IabB_0=(mga)/2`
`I=(mg)/(6bB_0)`
Magnetic force on wire `RS` is
`F=I((IxxB)`
`=I[(_bhatj)xx{(3hati+4hatk)B_0}]`
`F=IbB_0(3hatk-4hati)`