Let us suppose a bar magnet of length 2l is placed inside the magnetic field(B).
Torque(t) = Force acting × perpendicular distance(d).
Force acting =Bm
sinθ=d2l T=Bm×d
d=2lsinθ T=Bm.2lsinθ
M=m×2l T=MBsinθ
T=−→B×−→M or T=−→M×→B
Where ; B= Magnetic field of induction.
M= magnetic moment.
m= pole strength of bar magnet.
d= perpendicular distance.
2l= length of bar magnet.
T= torque.
Hence, the answer is −→M×→B.