The correct answer to the question is : Friction.
Step-by-step explanation:
Before answering this question, first we have to understand the process of charging a body by friction.
There are generally three modes of charging a body which are known as friction, induction and conduction respectively.
Friction: Friction is the mode of charging a body by direct contact through rubbing with another substance.
In this process, a low electron affinity substance is rubbed with a high electron affinity substance. The low electron affinity substance will lose electron, and acquires positive charge.
On the other hand, the high electron affinity substance will gain electrons, and gets negatively charged .
Hence, there is actual transfer of electrons from one body to another body in the process of charging a body through friction.
As per the question,the glass rod is rubbed with silk. Due this rubbing,static electricity will be developed on the glass rod and silk.
Hence, it is obvious that the glass rod gains static charge due to the process of friction.