In the dynamo-meter, the deflecting torque is directly proportional to the _______.

Correct option is B

 In a dynamometer type instrument, the deflecting torque is produced by the interaction of the magnetic fields created by the currents in the fixed and moving coils.

$$\text{Deflecting torque } (T_d) \text{ is given by} \\T_d = \text{Force} \times \text{perpendicular distance} \\T_d = (B I L N) \times b \\\text{Where,} \\B = \text{flux density in Wb/m}^2 \\L = \text{length or depth of coil in m} \\b = \text{breadth of the coil in m} \\N = \text{No. of turns in the coil} \\\text{Since, } L \times b = A \\\text{Where } A \text{ is area of the coil in m}^2 \\T_d = B I N A \\T_d \propto I$$​

 Therefore, the deflecting torque is proportional to the product of the currents flowing through both the fixed and moving coils.