The correct option is C rubbing
Insulator is a substance that has only few free electrons, such that these free electrons do not easily drift from one end of the substance to the other end when connected to a source of electricity.
An uncharged insulator can be charged by rubbing with another suitable body.
For example, on rubbing a glass rod with silk, the electrons from the glass rod get transferred to the silk. The silk now has an excess of electrons and so is negatively-charged. On the other hand, the glass rod is deficient in electrons and hence is positively-charged.