Introduction:
A PMMC (Permanent Magnet Moving Coil) meter is a type of instrument used for measuring direct current (DC) in electrical circuits. It operates on the principle of the interaction between a magnetic field produced by a permanent magnet and a current-carrying coil. The PMMC meter is widely used due to its accuracy, precision, and reliability.

Explanation:
The correct answer to the question is option 'B' - moving coil. Let's understand why.

Moving Coil:
The PMMC meter consists of a coil that is free to move within a magnetic field produced by a permanent magnet. The coil is wound on a lightweight frame and is connected in series with a controlling spring. When a current flows through the coil, it generates a magnetic field, and the interaction between this magnetic field and the fixed magnetic field of the permanent magnet causes a torque to be exerted on the coil.

Working Principle:
The working principle of a PMMC meter is based on Ampere's force law, which states that when a current-carrying conductor is placed in a magnetic field, a force is exerted on the conductor. This force is given by the equation F = BIL, where F is the force, B is the magnetic field, I is the current, and L is the length of the conductor.

Key Points:
- The moving coil in a PMMC meter acts as the current-carrying conductor.
- The permanent magnet provides a fixed magnetic field.
- The torque produced on the coil is proportional to the product of the current and the magnetic field strength.
- The controlling spring provides a restoring force to bring the coil back to its equilibrium position when the current is removed.
- The deflection of the coil is proportional to the current passing through it.
- The deflection is indicated by a pointer on a calibrated scale, allowing the measurement of the current.

Advantages of PMMC Meter:
- High accuracy and precision.
- Linear scale.
- Low power consumption.
- Wide operating range.
- Good reliability and long lifespan.

Conclusion:
In summary, a PMMC meter uses a moving coil as the current-carrying conductor. This moving coil interacts with a fixed magnetic field produced by a permanent magnet, and the resulting torque is used to measure the current. The PMMC meter is widely used due to its accuracy, precision, and reliability in measuring DC currents.