The sensitivity of a moving coil galvanometer increases with the decrease in
(A) Number of turns
(B) Area of coil
(C) Magnetic field
(D) Couple per unit twist

Hint:Sensitivity of galvanometer is defined as the current in micro ampere required to consume one-millimeter deflection on a scale placed 1 m away from the mirror. Galvanometer is a device which is used to measure very small current in the circuit. Also, it can be converted into ammeter as well as voltameter. The moving coil galvanometer works on the principle of deflection of the coil when current passes through it in presence of an external magnetic field.

Complete answer:
The current through the coil is directly proportional to the angle of twist of the suspension known as the deflection. Sensitivity of a galvanometer is given by the formula,
$k=I\theta =CNBA$
where A is the area of the coil, B is the external field, C is the torque.
Thus, we can see that sensitivity is directly proportional to the number of turns, area of the coil, and the torque. So, current sensitivity of a galvanometer can be increased by increasing the value of B, N and A. It can be decreased by decreasing the value of k.

So, the correct answer is D.

Note: Deflection of the coil is directly proportional to the current flowing through it. Hence, we use a linear scale in the galvanometer to detect the current flowing in the current. We know the coil moves under the influence of a magnetic field when current passes through it, so using this condition the torque experienced by the coil is given by, \[\tau =BINA\sin \theta \]. While putting all the quantities we have to bear in mind that they are in standard SI units.