Question

Why torque is maximum at 90 degrees ?

Answer 1

The above answer is correct.
Torque is the cross product of distance between point of application of force and center and force
= r * F * sin (theta)

Here maximum value of torque is r*F because we know sin(theta) has maximum value of 1....where theta =90 degree.

Hence torque is maximum when theta = 90 degree.
Hope this helps :)