Final answer:
A PMMC galvanometer can be converted for AC use by adding diodes that rectify the AC into DC, allowing the meter to provide a meaningful measurement.
Step-by-step explanation:
The PMMC (Permanent Magnet Moving Coil) galvanometer is designed primarily for direct current (DC) measurements. However, in order to use it for alternating current (AC), we need to convert the AC into DC. This is accomplished with the use of a rectifier. A rectifier typically contains diodes, which allow current to flow in only one direction, thus converting AC to DC.
When an AC is applied to a PMMC galvanometer without modification, the meter would try to move first in one direction and then in the opposite direction as the current alternates, resulting in no useful measurement. However, when introducing diodes, the AC is rectified, meaning it can then be read by the galvanometer as a steady DC. Adding a simple diode could suffice for half-wave rectification, but a bridge rectifier, which uses several diodes, can provide full-wave rectification and therefore a more stable and accurate measurement.
Therefore, the correct answer to the question is A) Diodes.