Correct Answer: (a) Copper
Solution:Karddamaka ruler's coinage can be categorised into two categories. 1st are those silver coins, which were quite frequent during the kshatrapas rule. 2nd are potin coins. Karddamaka ruler Chastana issued both silver & copper coin. Question is asked about rare coins, so correct answer will be option (a) copper, as copper coins were quite rare during Karddamaka rulers reign.