The correct answer is: B. Commutator motor meters
Commutator motor meters are not used on D.C. circuits because they require a rotating magnetic field to operate. This rotating magnetic field is created by the current flowing through the coils of the meter. However, in a D.C. circuit, the current only flows in one direction. This means that the magnetic field would not rotate, and the meter would not be able to operate.
Mercury motor meters and induction meters can be used on D.C. circuits. Mercury motor meters use a small amount of mercury to create a rotating magnetic field. Induction meters use a coil of wire to create a magnetic field. The magnetic field from the coil interacts with the magnetic field from the current flowing through the meter, causing the meter to rotate.
None of the above is not the correct answer because it is not one of the options.