The correct answer is C. proportional to the current.
The torque produced in an induction type relay is proportional to the current flowing through the coil. This is because the current creates a magnetic field, which interacts with the magnetic field of the stator to produce a torque. The torque is proportional to the current because the magnetic field is proportional to the current.
The other options are incorrect because they do not take into account the relationship between the current and the magnetic field. Option A is incorrect because the torque is not inversely proportional to the current. Option B is incorrect because the torque is not inversely proportional to the square of the current. Option D is incorrect because the torque is not proportional to the square of the current.