Torque is acting on a magnet, held at an angle $\theta $ with a magnetic field is maximum, when $\theta $ is
A. ${0^ \circ }$
B. ${180^ \circ }$
C. ${360^ \circ }$
D. ${90^ \circ }$

Hint: Torque is the measure of the force that can cause an object to rotate about an axis. Force is what causes an object to accelerate in linear kinematics. Similarly, torque is what causes an angular acceleration. Hence, torque can be defined as the rotational equivalent of linear force.

Complete step by step answer:
The point where the object rotates is called the axis of rotation. In physics, torque is simply the tendency of a force to turn or twist. Different terminologies such as moment or moment of force are interchangeably used to describe torque.

Above diagram shows the $\theta $, torque acting at an angle
Therefore,$\tau = \mathop M\limits^ \to \times \mathop B\limits^ \to $
Here $M$ is the Magnetic moment and $B$ is the Magnetic field.
From those we will the $\theta $ value, the product of magnetic moment and magnetic field is,
$\tau = MB\sin \theta $
$\sin \theta $ is maximum at $\theta = {90^ \circ }$.
Torque acting on a magnetic field is ${90^ \circ }$.

Hence, the correct answer is option D.

Note:Torque is defined as the product of force and the perpendicular distance between the force acting point and the pivot point. Torque in an electric medium means force experienced in an electric field, similarly, torque in a magnetic medium is the force experienced in a magnetic field.