Errors Associated and Compensation

Q. For a certain dynamometer ammeter the mutual inductance M varies with deflection $\theta$ (expressed in degree) as
$M=-6cos(\theta +30°)mH$

The deflecting torque produced by a direct current of 50 mA corresponding to a deflection of 60° is

1. $25\mu N-m$
2. $15\mu N-m$
3. $30\mu N-m$
4. $10\mu N-m$