Final answer:
Insulating materials can be charged by friction by gaining electrons when rubbed against another material. This electron transfer results in the insulator becoming negatively charged.
Step-by-step explanation:
Insulating materials can be charged by friction when they are rubbed against another material. During this process, electrons are transferred from one material to another. The material that gains electrons becomes negatively charged, while the material that loses electrons becomes positively charged. This occurs because different materials have varying affinities for electrons; the material with a greater affinity will gain electrons. Ultimately, the correct answer to the question of how insulating materials can be charged by friction is that they gain electrons through rubbing.
For example, when you rub your feet on the carpet and then touch a door handle, you may experience a shock due to the excess charge transferring from you to the conductive material of the handle. This example illustrates the transfer of electrons by the action of rubbing an insulator against another surface.