In moving-iron (MI) instruments, damping is a crucial factor for stabilizing the pointer so that it quickly comes to rest at its correct position without oscillating too much. Let's analyze the different types of damping options mentioned in the question:

1. Air Friction: In many measuring instruments, especially those of the moving-iron type, air friction damping is utilized. This is achieved by the movement of the pointer or a part associated with the movement through air, effectively using the air resistance to dampen oscillations. This method is simple and does not introduce any additional field interference, which makes it ideal for MI instruments. This is the correct statement regarding the damping in moving-iron instruments.
2. Eddy Currents: Eddy current damping involves the use of eddy currents induced in a metal (such as an aluminum disc) that is subjected to a magnetic field. This type of damping is more common in moving-coil instruments, not moving-iron instruments, due to the construction and operation principles involved. Therefore, this option is incorrect for MI instruments.
3. No Damping Required: While the pointer in MI instruments might be light, this does not eliminate the need for damping. Without adequate damping, the pointer would take longer to settle, leading to inefficiency and inaccuracy in readings. Consequently, this statement is also incorrect.
4. Permanent Magnet: Damping using a permanent magnet is another form of eddy current damping. This again is more applicable to moving-coil instruments rather than moving-iron instruments. It involves a magnetic field interacting with a conductive component to produce damping, which is not typical for MI instruments.

Thus, the correct statement is: "Damping in MI instruments is usually provided by air friction." This method is simple, effective, and does not interfere with the instrument's operation, making it suitable for moving-iron instruments.