Why are disc, rod and leaves of the electroscope made from metal?​

Correct option is:

B)Brass

Since charge gets transferred from the metal disk to the gold leaves through the metallic rod in a gold leaf electroscope. So, the rod has to be metallic to transfer the charge. Out of given options, wood, glass, rubber are insulators and therefore cannot be used. So, Brass is metal alloy and therefore used.

frequency or wavelenth of em radiation in a vacuum