A magnet of magnetic moment $50\hat iAm^2$ is placed along the X-axis in a magnetic field $\vec B=(0.5\hat i+3.0\hat j) T$. The torque acting on it is

$150\hat k\, Nm$

The correct answer is Option (2) → $150\hat k\, Nm$

The torque ($τ$) acting on a magnetic dipole is,

$τ=\vec m×\vec B$

$=(50\hat i)×(0.5\hat i+3.0\hat j)$

and,

$\hat i×\hat i=0$

$\hat i×\hat j=k$

$τ=50×[\hat i(0.5\hat i+3.0\hat j)]$

$=50×3\hat k$

$=150\hat kNm$